diff --git a/frontend/src/arthur/audio/mix.cljs b/frontend/src/arthur/audio/mix.cljs
index f97891a..f03710c 100644
--- a/frontend/src/arthur/audio/mix.cljs
+++ b/frontend/src/arthur/audio/mix.cljs
@@ -192,6 +192,23 @@
                (.arrayBuffer response)))
       (.then decode-bytes!)))
 
+(defn fit-buffer
+  "Fit fallback audio to the open timeline, padding with silence or trimming.
+  The buffer and clock must share a duration so looping wraps at the timeline's
+  end rather than repeating a short soundtrack underneath a longer animation."
+  [^js buffer seconds]
+  (let [rate (.-sampleRate buffer)
+        frames (max 1 (js/Math.ceil (* seconds rate)))]
+    (if (= frames (.-length buffer))
+      buffer
+      (let [channels (.-numberOfChannels buffer)
+            fitted (.createBuffer (js/OfflineAudioContext. channels 1 rate)
+                                  channels frames rate)]
+        (dotimes [c channels]
+          (.set (.getChannelData fitted c)
+                (.subarray (.getChannelData buffer c) 0 (min frames (.-length buffer)))))
+        fitted))))
+
 (defn clock-source!
   "Promise of `{:buffer :seconds}` — the audio the transport runs its clock on
   while symbol `sid` is open, and how long that clock is.
@@ -216,7 +233,9 @@
 
                  (and fallback-url (= sid (clip/opens-on document)))
                  (-> (decode! fallback-url)
-                     (.then (fn [^js b] {:buffer b :seconds (.-duration b)})))
+                     (.then (fn [b]
+                              (let [seconds (/ (clip/output-frames document sid) (:fps document))]
+                                {:buffer (fit-buffer b seconds) :seconds seconds}))))
 
                  :else
                  {:buffer nil
@@ -236,12 +255,8 @@
   symbol played against the whole take's soundtrack would run ten frames and then
   keep the clock going for minutes."
   [document sid fallback-url store]
-  (-> (buffer! document sid store)
-      (.then (fn [buffer]
-               (cond
-                 buffer (wav-url buffer)
-                 (and fallback-url (= sid (clip/opens-on document))) fallback-url
-                 :else (let [rate 44100
-                             n (max 1 (js/Math.ceil (* rate (/ (clip/output-frames document sid)
-                                                               (:fps document)))))]
-                         (wav-url (.createBuffer (js/OfflineAudioContext. 1 1 rate) 1 n rate))))))))
+  (-> (clock-source! document sid fallback-url store)
+      (.then (fn [{:keys [buffer seconds]}]
+               (wav-url (or buffer
+                            (.createBuffer (js/OfflineAudioContext. 1 1 44100)
+                                           1 (max 1 (js/Math.ceil (* 44100 seconds))) 44100)))))))
diff --git a/frontend/test/arthur/audio/mix_test.cljs b/frontend/test/arthur/audio/mix_test.cljs
new file mode 100644
index 0000000..d169575
--- /dev/null
+++ b/frontend/test/arthur/audio/mix_test.cljs
@@ -0,0 +1,25 @@
+(ns arthur.audio.mix-test
+  (:require [arthur.audio.mix :as mix]
+            [cljs.test :refer [deftest is]]))
+
+(defn- buffer [samples]
+  (let [data (js/Float32Array. (clj->js samples))]
+    #js {:sampleRate 12 :length (count samples) :numberOfChannels 1
+         :getChannelData (fn [_] data)}))
+
+(deftest fallback-audio-follows-the-timeline-length
+  (let [original (.-OfflineAudioContext js/globalThis)]
+    (set! (.-OfflineAudioContext js/globalThis)
+          (fn [_ _ _]
+            #js {:createBuffer (fn [_ frames _] (buffer (repeat frames 0)))}))
+    (try
+      (let [short (buffer (repeat 92 0.5))
+            fitted (mix/fit-buffer short (/ 5008 12))
+            data (.getChannelData fitted 0)]
+        (is (= 5008 (.-length fitted)))
+        (is (= 0.5 (aget data 91)))
+        (is (= 0 (aget data 92)))
+        (is (= 0 (aget data 5007)))
+        (is (identical? short (mix/fit-buffer short (/ 92 12))))
+        (is (= 20 (.-length (mix/fit-buffer short (/ 20 12))))))
+      (finally (set! (.-OfflineAudioContext js/globalThis) original)))))