Does the same happen on the other output jack?
Did you use ultrinit -o ?

Line out = Stereo ... whitout the use of ultrinit -o. My autoexect.bat have ultrinit -dj (not really certain of what it mean ... ) I guess i could use the line out ...
Seaker out = Mono 😮

It's happen often that GUS only output mono from speaker out?

I never really used GUS MAX myself, but it should be stereo on speaker out, so something is broken. Because both channels work on line out, it's got to be the power amp chip, or connector itself, or some component that the signal does not pass through. Or broken copper trace on PCB. Now, you need to figure out up to which point the signal goes fine and where it stops. Look for missing components or scratch marks perhaps, comparing to pictures found on the net, etc.

I will take some picture of the card, it is in pretty good shape and nothing seem to be missing. I thinking about rust or something in or on the connector ... anyway i can alway route the line out to the line in of my awe 32 and use them like that ...

Did you give it the ole' wiggle? Perhaps the speaker output has been used and abused. Ultrinit -dj = Disable Joystick I believe. If the right channel really gives you nothing and mixer settings are ok, perhaps it's an op-amp. Does using WIndows Sound System give you both channels btw?

🤣 Omg ... wich game use WSS? And the Ultrinit -o is for? I think i've this card like this since the begining. But i dont think the guy who swap this to me know the speak out is problematic. In did'nt wiggle the jack much, it is pretty tight and well soldered. 🙁 I hope isnt something serious ... i'm bad at soldering. In anycase i think i've some troubler using the line in of my AWE32 ... fml.

You can also look games that have Gravis Ultrasound and Gravis Ultrasound MAX options. The MAX option uses the WSS compatible codec on the board. If that gives you stereo sound we can narrow it down.

I'm still searching for a WSS game to use ... try in Tyrian 2000 but no sound at all ... i think is because i've misconfigured it someway. 🤣 I've routed the MT-32 through the line in of the GUS, and the line out of the GUS trought the AWE32. I finaly get the line in of the AWE32. And i finaly resolved the issu with the dos line in troubled awe32 (and i feel stupid about it, didnt have saved when i've check the box of what to output.) I will still continue to troubleshoot the poor thing ... at least i can use both chanel with the line out ...

EDIT : And in Tyrian 2000 my gus sound like a cancer ... dont know why, Sound great in every other game i can use it.