flat5

I was thinking it might help the random effect - provide the drive current - and it's been many years, but, don't some segment drivers have an enable pin? This would allow the blinking along with sound (music) level.

It might be a mess to breadboard, but the pcb would look clean :-)

You don't need to alter the soundcard so much. Just tack solder a light cable to two pads on the card. You can find a large ground area for the shield and a perhaps the line output pad for signal. I would run this to a cmos comparator.

I have not fooled around with this stuff since the 70s - early 80s.
Designed an octive divider for saxophone (that worked).