Just one question: what is the link between audio stuff and graphic card?
One claim is, that shared memory could cause issues. I own a card with it's own memory and currently I use an onboard device, that does share RAM for frame buffer. I can't confirm this claim. Shared memory seemingly never caused issues for me.
Sometimes when getting xruns (audio glitches), they can go away when using another graphics or graphics driver.
Using proprietary drivers sometimes doesn't work with real-time patched kernels, regarding to licensing issues. There are workarounds to ignore the license, that often do not work. Instead of using a real-time kernl, it's possible to use a full preempted kernel and as boot option using "threadirqs", however, a real-time patched kernel still could be a little bit better than a full preempted and since the hardware registers aren't accessible directly by PCs, as it is possible for the C64, Atari ST, PC anyway don't have that good real-time abilities, so IMO it's better to use a rt-patched kernel.
Last but not least, as for all hardware, the graphics could share the IRQ with the sound device, fortunately modern kernels seem to ensure that the graphics get it's own IRQ.
On my machine 2 USB ports, the graphics and my sound card share IRQ 18. With modern kernels, the graphics gets another IRQ and I wrote a script to unbind the USB ports.
Regards, Ralf