mocho
Sorry for the very late reply...
I have found a workaround. Nasty, but it works.
It turns out transcode sound only does not work if the source sound is AAC. Also see
tvheadend/tvheadend1663
So I changed the channel itself to already transcode using a ffmpeg command, transcoding it to mp3 using mux url:
pipe:///usr/bin/ffmpeg -loglevel fatal -i IPTVSOURCE -vcodec copy -acodec libmp3lame -b:a 128k -f mpegts -tune zerolatency pipe:1
Now the "source" audio is mp3, transcoding works with sound as well.
I use a IPTV source and therefore I can use the ffmpeg pipe command, I do not know how this should work for other inputs...