Audacity can do this without FFmpeg. It is more complicated to use FFmpeg to write U-Law.
If the track is stereo, Tracks > Stereo Track to Mono.
Set Project Rate bottom left to 8000 Hz.
File > Export Audio… then choose “Other uncompressed files”.
Click Options… .
Header should be “WAV (Microsoft)”.
Encoding should be “U-Law”. U-Law sets the 8-bit depth. Together with specifying 8000 Hz as above, this now gives you 64 kbps (bit rate of uncompressed files is bit depth * number of channels * sample rate).