I've had a problem for years with my Thinkpad 600X where my Roland SCP-55 won't output anything through the internal speakers if I connect it to the line-in jack.

Nothing I tried would enable the line-in and mix the incoming Roland audio with the onboard fm/digital sound through the speakers.

Just today I was messing around swapping over the Roland card between laptops and pushed the line-out from the Roland into the wrong jack on the laptop and I suddenly got that lovely SC-55 music playing through the onboard speakers... but I couldn't quite believe just how it was working...

https://youtu.be/L0nJ3BVuSPw

If I partially insert the 3.5mm jack from the SCP-55 into the headphone OUT of the Thinkpad, it plays perfectly! Line-in and Mic-in ports do nothing in DOS, which lead me to the suspicion that they are connected to the AC97 audio codec (this is one of those devices that has two audio chips: CS4297A AC97 codec and a CS4624 with legacy SB Pro functionality in DOS).

Of course it really isn't useable to have to balance the 3.5mm jack plug partially in the (headphone!) out socket... so it's still not a solution, but I wondered if anyone else had encountered similar wierd behaviour on a 600X, or other model with the dual CS4297A/CS4624 combo?

This machine is so close to being a great all round DOS/Win98 mobile gaming option, but the line-in functionality just breaks it completely for my purposes!