For the emulator, just google "no$gba" - That's the best DS emulator there is, just make sure you get the latest version. For the platinum rom, I don't really remember where I got it, but that's not too hard to find, either, you just google it.
You do have to correct some settings under "Emulation Setup", though. I took a snapshot for you, cos' I didn't want to write it all down - Just make sure you have these settings:
And yes, the sound output has to be "Digital Stereo", you can't turn it off. I just turn my speakers off, and put on a CD, but of course, that's up to you.
I was just wondering. Where did you get your Platinum rom and DS emulator? The ones I got kept showing some Japanese writing on a blue background when I opened it and nothing else would happen after that.