Can you mute the microphone input, there's usually a check box for this, and then try increasing the line-in level if you have one. Sometimes laptops, and some sound cards, have a single input socket for different uses. See
Flexijack.
At least, as you say, you can now get some sort of recording which is a start.
If you can't enhance the volume from the software or the mixer you could use a freeware audio editor like Audacity and boost it that way.
