No sound when converting audio to AC3 (Aften) in Avidemux

[spapakons]

I use Avidemux to convert a video to a smaller version (640x480 HEVC, AC3 mono) so I can transfer to my smartphone for viewing outside home. I don't know if it is related, but since I upgraded K-Lite codecs a couple of times I noticed a weird behaviour. Selecting to convert audio to AC3 (Aften) produces a muted video. Selecting to convert with AC3 (Lame) and same bitrate audio is normal. I didn't update Aften (latest version) and as far as I know neither does K-Lite codecs Mega. Anyone knows why the audio is muted? I only select to downmix from Stereo to Mono, with the original sampling rate (48000Hz) and the original precision (16-bit). I don't enable any other audio filters.

[ProWo]

Retry with 8 bit instead of 16.

[spapakons]

And make it sound like a robot? No thanks! I would rather export the audio and convert it with Adobe Audition.