If you're using an older case with newer mobo, make sure that you have the right front panel audio (FP_AUDIO) connector. Intel started using HD_Audio connectors instead of AC'97 around or before the times of Core2Duo. Both are 2x5 pin headers, so easy to mix up. If a HD_Audio connector is unknowingly connected to an AC'97 header or cable, it can have unintended consequences. Sparks or explosions unlikely, but (for example) plugging a pair of headphones into the headphone jack may not mute the speakers as expected.
This article discusses the matter:
https://superuser.com/questions/419038/hd-audio-or-ac97-connector-which-to-use-when-and-whats-the-difference, and there is plenty more if you do an internet search.
You may also want to check your BIOS settings.
As you reported no problems using Retroarch, maybe it is software, check sound settings in your mame.ini. "portaudio" has been known to cause some issues.
Re: doubling down on lilshawn's comments, I still use Core2Duo machines with 775 mobos and E7X00 - E8600 CPUs for mame - I wouldn't bother with the quad core CPUs. A E8500 will give you better mame performance than a Q9550.
The E8XXX CPUs are very good performers in general and very cheap these days, although the E8600 3.33Ghz is a bit harder to find as it is the top of the line (though I've heard about an unofficial E8700, but rare like hen's teeth). As a testament to their single-core performance and durability, the E8600 are (were?) popular with overclockers, presumably adding to their rarity (wonder how many have been burned up?).
EDIT: comparing E8500 with Q9550, they both have very similar single core and dual-core performance, however the E8500 is rated at 65W vs the Q9550 rated at 95W. So the Q9550 uses nearly 50% more power to do the same thing as the E8500, produces more waste heat too.
EDIT2: I've overclocked an E8600 to over 4Ghz and was able to run Gauntlet Legends at "full speed" with no latency/stutter. Could have gone much higher apparently, but that was just one-time for the giggles, generally I don't overclock CPUs.