diff --git a/.gitattributes b/.gitattributes
index a4a7a72..bdbda26 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -4,3 +4,5 @@
 *.jpeg filter=lfs diff=lfs merge=lfs -text
 *.blend filter=lfs diff=lfs merge=lfs -text
 *.blend1 filter=lfs diff=lfs merge=lfs -text
+*.wav filter=lfs diff=lfs merge=lfs -text
+*.mp3 filter=lfs diff=lfs merge=lfs -text
diff --git a/assets/engine_loop.wav b/assets/engine_loop.wav
new file mode 100644
index 0000000..4651c2b
--- /dev/null
+++ b/assets/engine_loop.wav
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:97f8d472d4ef3990bec9d2cf209ce06531c6db767925bace1e7a1b4811d0e903
+size 234818
diff --git a/game/assets/assets.odin b/game/assets/assets.odin
index ec762d6..f35854e 100644
--- a/game/assets/assets.odin
+++ b/game/assets/assets.odin
@@ -49,6 +49,8 @@ Asset_Manager :: struct {
 	textures:  Asset_Cache(rl.Texture2D),
 	models:    Asset_Cache(rl.Model),
 	shaders:   Asset_Cache(Loaded_Shader),
+	sounds:    Asset_Cache(rl.Sound),
+	music:     Asset_Cache(rl.Music),
 	curves_1d: Asset_Cache([]f32),
 	curves_2d: Asset_Cache(Curve_2D),
 	bvhs:      map[cstring]Loaded_BVH,
@@ -107,6 +109,26 @@ SHADER_LOADER :: Asset_Cache_Loader(Loaded_Shader) {
 	},
 }
 
+SOUND_LOADER :: Asset_Cache_Loader(rl.Sound) {
+	load = proc(path: cstring, payload: Asset_Cache_Loader_Payload) -> (rl.Sound, bool) {
+		sound := rl.LoadSound(path)
+		return sound, rl.IsSoundValid(sound)
+	},
+	unload = proc(sound: rl.Sound) {
+		rl.UnloadSound(sound)
+	},
+}
+
+MUSIC_LOADER :: Asset_Cache_Loader(rl.Music) {
+	load = proc(path: cstring, payload: Asset_Cache_Loader_Payload) -> (rl.Music, bool) {
+		music := rl.LoadMusicStream(path)
+		return music, rl.IsMusicValid(music)
+	},
+	unload = proc(music: rl.Music) {
+		rl.UnloadMusicStream(music)
+	},
+}
+
 MODEL_LOADER :: Asset_Cache_Loader(rl.Model) {
 	load = proc(path: cstring, payload: Asset_Cache_Loader_Payload) -> (rl.Model, bool) {
 		model := rl.LoadModel(path)
@@ -176,6 +198,12 @@ assetman_init :: proc(assetman: ^Asset_Manager) {
 	assetman.shaders = {
 		loader = SHADER_LOADER,
 	}
+	assetman.sounds = {
+		loader = SOUND_LOADER,
+	}
+	assetman.music = {
+		loader = MUSIC_LOADER,
+	}
 	assetman.textures = {
 		loader = TEXTURE_LOADER,
 	}
@@ -321,6 +349,16 @@ get_shader :: proc(
 	return loaded_shader
 }
 
+get_sound :: proc(assetman: ^Asset_Manager, path: cstring) -> rl.Sound {
+	sound, _, _ := assetcache_fetch_or_load(&assetman.sounds, path)
+	return sound
+}
+
+get_music :: proc(assetman: ^Asset_Manager, path: cstring) -> rl.Music {
+	sound, _, _ := assetcache_fetch_or_load(&assetman.music, path)
+	return sound
+}
+
 null_bvhs: []bvh.BVH
 
 get_bvh :: proc(assetman: ^Asset_Manager, path: cstring) -> Loaded_BVH {
@@ -784,6 +822,8 @@ shutdown :: proc(assetman: ^Asset_Manager) {
 	assetcache_destroy(&assetman.textures)
 	assetcache_destroy(&assetman.models)
 	assetcache_destroy(&assetman.shaders)
+	assetcache_destroy(&assetman.sounds)
+	assetcache_destroy(&assetman.music)
 	assetcache_destroy(&assetman.curves_1d)
 	assetcache_destroy(&assetman.curves_2d)
 
diff --git a/game/game.odin b/game/game.odin
index 58f066b..efff1a3 100644
--- a/game/game.odin
+++ b/game/game.odin
@@ -361,6 +361,7 @@ World_Update_Config :: struct {
 }
 
 update_world :: proc(world: ^World, dt: f32, config: World_Update_Config) {
+
 	if !world.pause {
 		car_model := assets.get_model(&g_mem.assetman, "assets/ice_cream_truck.glb")
 		car_bounds := rl.GetModelBoundingBox(car_model)
@@ -626,7 +627,7 @@ update_world :: proc(world: ^World, dt: f32, config: World_Update_Config) {
 				axle = physics.Drive_Axle_Config {
 					wheels = {wheel_rl, wheel_rr},
 					wheel_count = 2,
-					diff_type = .Open,
+					diff_type = .Fixed,
 					final_drive_ratio = 4.1,
 				},
 			},
@@ -747,6 +748,24 @@ update_world :: proc(world: ^World, dt: f32, config: World_Update_Config) {
 			step_mode = config.single_step_physics ? physics.Step_Mode.Single : physics.Step_Mode.Accumulated_Time,
 		)
 	}
+
+	{
+		engine_loop := assets.get_music(&g_mem.assetman, "assets/engine_loop.wav")
+
+		if !rl.IsMusicStreamPlaying(engine_loop) {
+			rl.PlayMusicStream(engine_loop)
+		}
+		rl.UpdateMusicStream(engine_loop)
+
+		sim_state := physics.get_sim_state(&world.physics_scene)
+		engine := physics.get_engine(sim_state, world.engine_handle)
+		rpm_percent :=
+			max(physics.angular_velocity_to_rpm(engine.w) - engine.lowest_rpm, 0.0) /
+			(engine.rev_limit_rpm - engine.lowest_rpm)
+
+		pitch := linalg.lerp(f32(1.0), f32(3.0), rpm_percent)
+		rl.SetMusicPitch(engine_loop, pitch)
+	}
 }
 
 update_runtime_world :: proc(runtime_world: ^Runtime_World, dt: f32) {
@@ -1393,6 +1412,7 @@ game_init_window :: proc(args: []string) {
 
 	rl.SetConfigFlags({.WINDOW_RESIZABLE, .VSYNC_HINT})
 	rl.InitWindow(1280, 720, "Gutter Runner")
+	rl.InitAudioDevice()
 	rl.SetExitKey(.KEY_NULL)
 	rl.SetWindowPosition(200, 200)
 	rl.SetTargetFPS(120)
@@ -1441,6 +1461,7 @@ game_shutdown :: proc() {
 
 @(export)
 game_shutdown_window :: proc() {
+	rl.CloseAudioDevice()
 	rl.CloseWindow()
 
 	deinit_physfs()
diff --git a/game/physics/collision/convex.odin b/game/physics/collision/convex.odin
index 18c3496..a38b04f 100644
--- a/game/physics/collision/convex.odin
+++ b/game/physics/collision/convex.odin
@@ -77,6 +77,8 @@ double_sided_triangle_to_convex :: proc(
 	convex.vertices[0].pos = tri[0]
 	convex.vertices[1].pos = tri[1]
 	convex.vertices[2].pos = tri[2]
+	convex.faces[0].normal = lg.normalize0(lg.cross(tri[1] - tri[0], tri[2] - tri[0]))
+	convex.faces[1].normal = -convex.faces[0].normal
 	return
 }
 
diff --git a/game/physics/simulation.odin b/game/physics/simulation.odin
index 9ecfab3..54b43d0 100644
--- a/game/physics/simulation.odin
+++ b/game/physics/simulation.odin
@@ -896,10 +896,10 @@ pgs_solve_contacts :: proc(
 			random_order[i] = i32(i)
 		}
 
-		// for i in 0 ..< len(random_order) - 1 {
-		// 	j := rand.int_max(len(random_order))
-		// 	slice.swap(random_order, i, j)
-		// }
+		for i in 0 ..< len(random_order) - 1 {
+			j := rand.int_max(len(random_order))
+			slice.swap(random_order, i, j)
+		}
 	}
 
 	for i in 0 ..< len(sim_state.contact_container.contacts) {
@@ -1027,7 +1027,7 @@ lookup_gear_ratio :: #force_inline proc(gear_ratios: []f32, gear: i32) -> (ratio
 			index -= 1
 		}
 		index = clamp(index, 0, len(gear_ratios) - 1)
-		return gear_ratios[index]
+		return gear_ratios[index] * (index == 0 ? -1 : 1)
 	}
 }