Oops - I posted too soon. I was meant to link this Github issue: https://github.com/rg3/youtube-dl/issues/8070...
UnicodeDecodeError: 'utf8' codec can't decode byte 0xb6 in position 147: invalid start byte
Indeed, cycles=8000 seems to circumvent this issue. I had cycles=max. Thanks!
Silverball makes Dosbox hang