A "normal" GUS clearly is NOT the optimal target card for this player. Native support is always superior than emulation. GUS Classic has no native support in MPXPLAY.
Note that the GUS MAX has native support, and now I gave it a try for you, and it works, but the DX4/100 in my retro rig makes sound staccato at best. You need a P100 at least for this, I guess. I tested this with 128 kb/s mp3 files, 44100 Hz playback. At 22 kHz the CS4231 plays mp3 files fluid, but the sound is distorted.
With all these things said, it is still possible that with a beefy rig you wil get satisfactory results with SBOS or MegaEM. You might need to tinker with the ini file (pre-buffering etc.)
"Ita in vita ut in lusu alae pessima iactura arte corrigenda est."