Fall back to first audio track when none preferred

Auto-selection still prefers Japanese/Chinese/undefined audio, but
if a file has none of those (e.g. only English), the first audio
track is pre-checked instead of leaving no audio track selected.

Services/MkvService.cs

@@ -93,9 +93,12 @@ public class MkvService
 
     /// <summary>
     /// Builds the list of <see cref="TrackItem"/> objects for a file. Video
-    /// tracks are always pre-checked. Exactly one audio track is pre-checked:
-    /// among Japanese/Chinese/undefined-language tracks, a stereo (2-channel)
-    /// track is preferred, otherwise the first matching track in file order.
+    /// tracks are always pre-checked. Exactly one audio track is always
+    /// pre-checked: among Japanese/Chinese/undefined-language tracks, a
+    /// stereo (2-channel) track is preferred, otherwise the first matching
+    /// track in file order. If no such track exists (e.g. only English
+    /// audio), the very first audio track in the file is pre-checked instead,
+    /// so a file is never left with no audio track selected.
     /// Subtitle tracks are never pre-checked — the user picks one via the
     /// track table, and the choice propagates to other queued files.
     /// </summary>
@@ -108,7 +111,8 @@ public class MkvService
             .ToList();
 
         MkvTrack? preferredAudio = eligibleAudio.FirstOrDefault(t => t.Properties?.AudioChannels == 2)
-                                    ?? eligibleAudio.FirstOrDefault();
+                                    ?? eligibleAudio.FirstOrDefault()
+                                    ?? audioTracks.FirstOrDefault();
 
         var items = info.Tracks.Select(t =>
         {