Not sure about swiftfin but try the option downmix to stereo somewhere in the client playback settings

That will be transcoding so from the server side make sure it's enabled and working. Then you can limit the bitrate (per-user, or globally)

This way the client will stream the content and not direct play it.

Hopefully this fixes the issue with audio.

https://pine64.org/devices/pinenote/

Free hardware, free software

$399 on the global store they ship from Hong Kong and pricey

https://pine64.com/product-category/tablets/pinenote/