Documented the transcription bitrate decisions and hardened its fallback

app/MindWork AI Studio/Plugins/configuration/plugin.lua

@@ -582,6 +582,9 @@ CONFIG["SETTINGS"] = {}
 -- Allowed values are: KBPS_32, KBPS_64, KBPS_128, KBPS_256
 -- Higher bitrates improve transcription accuracy on noisy or quiet recordings, at the cost of
 -- a larger upload to the transcription provider. KBPS_128 is recommended.
+-- Please note: this bitrate applies whenever a recording has to be re-encoded. A file which already
+-- is a single mono 48 kHz Opus track in a WebM container and stays below 25 MiB is forwarded to the
+-- transcription provider unchanged, keeping the bitrate it was created with.
 -- CONFIG["SETTINGS"]["DataApp.OpusBitrate"] = "KBPS_32"
 --
 -- Allow the user to change the Opus bitrate even though your organization set a default above:

app/MindWork AI Studio/Settings/ConfigurationSelectDataFactory.cs

@@ -327,6 +327,8 @@ public static class ConfigurationSelectDataFactory
     public static IEnumerable<ConfigurationSelectData<TranscriptionOpusBitrate>> GetTranscriptionOpusBitrateData()
     {
         foreach (var bitrate in Enum.GetValues<TranscriptionOpusBitrate>())
+        {
             yield return new(bitrate.GetName(), bitrate);
         }
     }
+}

app/MindWork AI Studio/Settings/DataModel/DataApp.cs

@@ -115,6 +115,13 @@ public sealed class DataApp(Expression<Func<Data, DataApp>>? configSelection = n
     /// <summary>
     /// The Opus bitrate used when normalizing uploaded audio/video for transcription.
     /// </summary>
+    /// <remarks>
+    /// Every recording is re-encoded to mono Opus before it goes to the transcription provider. That
+    /// encoding used to be fixed at 32 kbps, which cost the transcription models entire quiet passages:
+    /// a greeting spoken softly at the start of a recording was simply missing from the transcript. The
+    /// same recording compared at 64 and 128 kbps came back complete, which is why 128 kbps is the
+    /// default here. Users trading accuracy for a smaller upload can still pick a lower bitrate.
+    /// </remarks>
     public TranscriptionOpusBitrate OpusBitrate { get; set; } = ManagedConfiguration.Register(configSelection, n => n.OpusBitrate, TranscriptionOpusBitrate.KBPS_128);
 
     /// <summary>

app/MindWork AI Studio/Settings/DataModel/TranscriptionOpusBitrate.cs

@@ -2,8 +2,13 @@ namespace AIStudio.Settings.DataModel;
 
 public enum TranscriptionOpusBitrate
 {
+    // The recommended bitrate is deliberately the member with the underlying value 0: when the
+    // settings file holds a value TolerantEnumConverter cannot read, it falls back to that member.
+    // Landing on the lowest bitrate there would silently reintroduce the very defect this setting
+    // exists to prevent -- transcripts losing what was said quietly.
+    KBPS_128 = 0,
+
     KBPS_32,
     KBPS_64,
-    KBPS_128,
     KBPS_256,
 }

app/MindWork AI Studio/Settings/DataModel/TranscriptionOpusBitrateExtensions.cs

@@ -21,6 +21,9 @@ public static class TranscriptionOpusBitrateExtensions
         TranscriptionOpusBitrate.KBPS_64 => 64_000,
         TranscriptionOpusBitrate.KBPS_128 => 128_000,
         TranscriptionOpusBitrate.KBPS_256 => 256_000,
-        _ => 32_000,
+
+        // A value we do not know must never mean the lowest quality: that is how quiet passages
+        // went missing from transcripts in the first place. Fall back to the recommended bitrate.
+        _ => 128_000,
     };
 }

app/MindWork AI Studio/Tools/Services/MediaTranscriptionService.cs

@@ -647,9 +647,14 @@ public sealed class MediaTranscriptionService(
         MediaOperation operation,
         bool updateImportState)
     {
+        // The bitrate governs re-encoding, not every upload: the runtime hands a file through
+        // unchanged when it already is a single mono 48 kHz Opus track in a WebM container and small
+        // enough. Such a file keeps whatever bitrate it was made with, which is the better outcome --
+        // re-encoding it could only take quality away, never add any.
+        var opusBitrateBps = settingsManager.ConfigurationData.App.OpusBitrate.GetBitsPerSecond();
+
         // The quick POST is intentionally not cancelled: losing its response could orphan a job
         // whose ID the client never received. Cancellation is applied immediately after ownership.
-        var opusBitrateBps = settingsManager.ConfigurationData.App.OpusBitrate.GetBitsPerSecond();
         var jobId = await rustService.StartMediaJobAsync(mediaPath, normalizedPath, opusBitrateBps, CancellationToken.None);
         operation.JobId = jobId;
 

runtime/src/media.rs

@@ -60,6 +60,11 @@ const OPUS_PRE_SKIP: u16 = 312;
 const DEFAULT_MAX_PASS_THROUGH_BYTES: u64 = 25 * 1024 * 1024;
 
 /// Default target bitrate for mono speech-oriented Opus output, used when a request omits it.
+///
+/// AI Studio always states a bitrate, so this default only covers requests which leave it out. It is
+/// 128 kbps because the 32 kbps this encoder used to be fixed at cost transcription models whole
+/// quiet passages: a softly spoken greeting at the start of a recording never reached the transcript,
+/// while the same recording at 128 kbps came back complete.
 const DEFAULT_OPUS_BITRATE_BPS: u32 = 128_000;
 
 /// Bounded input block used for streaming resampling.