Understandable, but keep in mind that upgrading to a newer OS will almost always result in a degradation in performance. Depending on your computer's hardware, that degradation might be so small that it's unnoticeable, or so noticeable that it's unusable. Newer operating systems more often than not have higher requirements when it comes to memory, processing power, graphics, etc., and that's the case whether you're talking about Windows, Mac, Linux. Sometimes, the new version is optimized to run better than the previous version on the same hardware (think Windows Vista vs. 7), but that's more of an exception than a rule.
That said, the easiest way to solve both the issue of the missing sound and of the missing performance would be to 1) backup your data and 2) restore back to factory settings using your installation DVDs (or, depending on your computer model, restoring through Internet Recovery). This would bring back the original software, which would contain the correct drivers (.kext files) for audio and the rest of the hardware, as well as installing the operating system for which your computer was designed (thus giving you back the performance of when it was out of the box). Sorry for the long post, but hope this helps!