The -b option (output buffer) fails with no error message with all versions in portage.Which includes: 0.59s-r11 0.59s-r9 However, this option works fine with an ubuntu binary of 0.59r.The two gentoo releases are both 64bit while the ubuntu binary is 32-bit.I will try to get hold of a 32bit release of this version in case that is the problem. You can reproduce this by running : mpg123 -b 1024 foobar.mp3
this seems fixed in mpg123-0.65. if you have problems with that version, please reopen and include emerge --info.