Hope someone is still around to maybe answer this!
I've compiled mjs and mpg123 with the provided pach on a Digital Hinote Ultra II running Slack 10.
When I try and run mjs with buffering turned on, it immediately kicks me out with mgs:error: starting mpg123 failed !
Running mpg123 from the command line with -b n>0 returns immediately to Decoding Finished. -b 0 plays normally though.
I built mpg123 with linux-pentium. gcc version 3.3.4.