Como Nifle comentou, você pode usar sed assim. Sem sed, se sua saída for:
$ ffmpeg -i input.avi -b:v 64k -bufsize 64k output.avi -loglevel debug
here is a line without any brackets and stuff, it should display too
[dca @ 0x7fe86c80f000] leave this stuff here
[dca @ 0x7fe86c80f000] and this
[dca @ 0x7fe86c80f000] this stuff too
another line that should just be printed plainly.
Então você pode adicionar
| sed 's/\[.*\] *\t*//'
para o final, assim: (correu no meu linux, o osX sed pode ser um pouco diferente)
$ ffmpeg -i input.avi -b:v 64k -bufsize 64k output.avi -loglevel debug | sed 's/\[.*\] *\t*//'
here is a line without any brackets and stuff, it should display too
leave this stuff here
and this
this stuff too
another line that should just be printed plainly.
Parece bom?