You may

ffmpeg -i input.mkv -c:v copy -c:a libfaac out.mp4

or if there is no joy, try fixing the internal bitstream

conversion to stereo and asc

ffmpeg -i input.mkv -c:v copy -c:a libfaac    \
-bsf:a aac_adtstoasc -ac 2 -ar 48000 -ab 256k \
out.mkv

If it is 5.1surround and the voices are hard to hear it is because the dialog is on the center channel and you are only hearing the off-mike from the R & L channels but the music and sound effects are booming.

You can compensate for this with dolby-II filter to get the surround channel cut way down and the center channel split into stereo, and combine them all into right and left stereo channels.

ffmpeg -i input.mkv -c:v copy -c:a libfaac -bsf:a aac_adtstoasc   \
  -af aresample=matrix_encoding=dplii -ac 2 -ar 48000 -ab 256k    \
  out.mkv

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

You may do this:

ffmpeg -i input.mkv -c:v copy -c:a libfaac out.mp4

Or if there is no joy, try fixing the internal bitstream

Conversion to stereo and asc:

ffmpeg -i input.mkv -c:v copy -c:a libfaac    \
-bsf:a aac_adtstoasc -ac 2 -ar 48000 -ab 256k \
out.mkv

If it is 5.1, surround and the voices are hard to hear it is because the dialog is on the center channel and you are only hearing the off-mike from the R & L channels but the music and sound effects are booming.

You can compensate for this with dolby-II filter to get the surround channel cut way down and the center channel split into stereo, and combine them all into right and left stereo channels.

ffmpeg -i input.mkv -c:v copy -c:a libfaac -bsf:a aac_adtstoasc   \
  -af aresample=matrix_encoding=dplii -ac 2 -ar 48000 -ab 256k    \
  out.mkv