Just saying it's a sempron is not very specific: http://en.wikipedia.org/wiki/List_of_AMD_Sempron_microprocessors
Some models are nearly 10 years old. If you've got one that old I'd say it's time for a new system.
It might also help if you provide more details of your ram (like speed and whether it's ddr2 or ddr3), what format of audio you're trying to play, and what program you're using to play it.