When running a lot of emulators, which includes No$GBA on DS mode, PCSx2 (Ocassionally), Dolphin and a few others, all the sound is playing but it is very scratchy, like UNBEARABLY NAILS ON CHALKBOARD DEAR GOD MAKE IT STOP scratchy. I've dicked with the settings, it's the worst on No$GBA. It doesn't matter which game I have loaded, it still dicks it in the butt.
See this wat do?
Since you haven't mentioned it, you should try testing out other mediums of listening to the sound? Try another speaker/headphones/earplugs/etc.
I've actually started to get this problem myself, but ever since I updated my video card drivers and updating DirectX. I actually can't find out what is causing it myself. It is very noticeable in Dolphin now.
Have you updated any of your hardware recently?
Record a sample and post?
It would also show how far along the chain the problem is. If recording from loopback / stereo mix is absent from the noise, you know it's beyond your soundcard.
How does I record sounds
Mind if I see your NO$GBA config?