How to resolve ffmpeg error "input contains (near) nan/+-inf"?
If you have a ffmpeg build with libfdk_aac then you can use that encoder since it doesn't report that error.

With your current build, you have to convert the audio to 16 bit integer samples first.

ffmpeg -i input.wmv -vf "scale=320:240,setsar=1" -crf 24 -af aformat=s16p -c:a aac -b:a 48k -ac 1 -movflags +faststart output.mp4