Hi Mike,
I do vaguely recall that the CM108B ICs have lower gain and do drop off or go to zero below about 400 on the rx/tx mix gain settings. (Issue is with the C-Media chip or ASL driver ie. shouldn’t have anything to do with the 3040 itself.) But if you keep the gain above that you should be fine and the actual dB gain range should still be the same vs CM1xx A/AH, despite the different mapping to the 0-999 mixset paramater ranges. Also make sure you have rxboost and txboost set to 0 (off) in the rpt and simpleusb / usbradio .conf files.
Re. using the internal sound (3040 headphone jack) yes that is possible using the ALSA driver, see this post for a couple links with details on how to do that. DRAWS hat with ASL? - #2 by NR9V I have not done this personally but have heard of some other people who successfully have made it work.