[Feature Request] Add audio channel downmix option for transcoding (5.1ch to stereo)

[KatsuhikoIgari]

I have searched the existing feature requests, both open and closed, to make sure this is not a duplicate request.

• Yes

The feature

TL;DR

Immich preserves multi-channel audio during transcoding.
When the source contains AC-3 5.1ch audio (e.g. MTS/AVCHD), the output becomes AAC 5.1ch,
which has no audio playback on iOS (Safari / WKWebView / Immich iOS app).

This is not a bug but a missing configuration option.

Adding an optional stereo downmix setting (FFmpeg -ac 2) would:

• Fix silent playback on iOS devices
• Improve mobile/web compatibility
• Reduce file size
• Keep current behavior as default

Proposed setting:
Administration → Settings → Video → Transcoding → Audio Channels

• Auto (preserve original)
• Stereo (2ch)
• 5.1 (6ch)

Evidence:

• AAC 5.1ch → ❌ iOS Immich app / Safari / Chrome (WebKit)
• AAC stereo (downmixed) → ✅ works everywhere

In short: add an optional audio channel downmix (stereo) option to improve iOS compatibility
for multi-channel source videos.

Platform

• Server
• Web
• Mobile