diff --git a/Sources/VideoIO/MultitrackMovieRecorder.swift b/Sources/VideoIO/MultitrackMovieRecorder.swift
index e673403..beeebc8 100644
--- a/Sources/VideoIO/MultitrackMovieRecorder.swift
+++ b/Sources/VideoIO/MultitrackMovieRecorder.swift
@@ -300,32 +300,60 @@ public final class MultitrackMovieRecorder {
                             audioSettings = [AVFormatIDKey : kAudioFormatMPEG4AAC]
                         }
                         
+                        // Populate sample rate and channel count from the format description.
+                        // Guard against zero values that can occur when an external USB audio
+                        // device is connected before app launch, causing AVCaptureSession to
+                        // initialise audio in a degraded state with an incomplete ASBD.
                         if let currentASBD = CMAudioFormatDescriptionGetStreamBasicDescription(formatDescription) {
-                            audioSettings[AVSampleRateKey] = currentASBD.pointee.mSampleRate
-                            audioSettings[AVNumberOfChannelsKey] = currentASBD.pointee.mChannelsPerFrame
+                            let sampleRate = currentASBD.pointee.mSampleRate
+                            let channelCount = currentASBD.pointee.mChannelsPerFrame
+                            audioSettings[AVSampleRateKey] = sampleRate > 0 ? sampleRate : Double(44100)
+                            audioSettings[AVNumberOfChannelsKey] = channelCount > 0 ? channelCount : UInt32(1)
+                        } else {
+                            // ASBD unavailable — fall back to safe mono defaults.
+                            audioSettings[AVSampleRateKey] = Double(44100)
+                            audioSettings[AVNumberOfChannelsKey] = UInt32(1)
                         }
-                        
+
+                        // Only set AVChannelLayoutKey when real layout data is present.
+                        // Setting it to Data() (empty) causes canApply(outputSettings:forMediaType:audio)
+                        // to return false, which throws cannotSetupAudioInputs. This happens when
+                        // CMAudioFormatDescriptionGetChannelLayout returns nil — e.g. when an external
+                        // USB audio device was connected before launch and AVCaptureSession initialised
+                        // audio without a channel layout. Per Apple docs, omitting AVChannelLayoutKey
+                        // is valid for ≤2 channels; AVAssetWriter infers mono/stereo from
+                        // AVNumberOfChannelsKey.
                         var aclSize: Int = 0
                         let currentChannelLayout = CMAudioFormatDescriptionGetChannelLayout(formatDescription, sizeOut: &aclSize)
-                        let currentChannelLayoutData: Data
                         if let currentChannelLayout = currentChannelLayout, aclSize > 0 {
-                            currentChannelLayoutData = Data(bytes: currentChannelLayout, count: aclSize)
-                        } else {
-                            currentChannelLayoutData = Data()
+                            audioSettings[AVChannelLayoutKey] = Data(bytes: currentChannelLayout, count: aclSize)
                         }
-                        audioSettings[AVChannelLayoutKey] = currentChannelLayoutData
-                        
-                        if self.assetWriter.canApply(outputSettings: audioSettings, forMediaType: .audio) {
-                            let audioInput = AVAssetWriterInput(mediaType: .audio, outputSettings: audioSettings, sourceFormatHint: formatDescription)
+
+                        func makeAndAddAudioInput(settings: [String: Any], hint: CMFormatDescription?) throws {
+                            let audioInput = AVAssetWriterInput(mediaType: .audio, outputSettings: settings, sourceFormatHint: hint)
                             audioInput.expectsMediaDataInRealTime = true
-                            if self.assetWriter.canAdd(audioInput) {
-                                self.assetWriter.add(audioInput)
-                                audioInputs.append(audioInput)
-                            } else {
+                            guard self.assetWriter.canAdd(audioInput) else {
                                 throw RecorderError.cannotSetupAudioInputs
                             }
+                            self.assetWriter.add(audioInput)
+                            audioInputs.append(audioInput)
+                        }
+
+                        if self.assetWriter.canApply(outputSettings: audioSettings, forMediaType: .audio) {
+                            try makeAndAddAudioInput(settings: audioSettings, hint: formatDescription)
                         } else {
-                            throw RecorderError.cannotSetupAudioInputs
+                            // Last resort: minimal known-good settings that AVAssetWriter always accepts
+                            // for AAC output, used only if the format-description-derived values are
+                            // still rejected (e.g. unsupported sample rate from a degraded device).
+                            let safeSettings: [String: Any] = [
+                                AVFormatIDKey: kAudioFormatMPEG4AAC,
+                                AVSampleRateKey: Double(44100),
+                                AVNumberOfChannelsKey: UInt32(1)
+                            ]
+                            guard self.assetWriter.canApply(outputSettings: safeSettings, forMediaType: .audio) else {
+                                throw RecorderError.cannotSetupAudioInputs
+                            }
+                            try makeAndAddAudioInput(settings: safeSettings, hint: nil)
                         }
                     }
                     self.audioInputs = audioInputs