You can do this with PowerShell using the `n
for a newline and optionally
`r
for carriage return
ffmpeg -i in.m4a -metadata comment="hello`nworld" out.m4a
Windows default shell is cmd.exe
so you may need to invoke like this
powershell ffmpeg -i in.m4a -metadata comment="hello`nworld" out.m4a
Output
Output #0, ipod, to 'out.m4a': Metadata: major_brand : M4A minor_version : 512 compatible_brands: isomiso2 comment : hello : world encoder : Lavf55.1.100