Weird sounds

I have a 2002 Honda Accord Coupe, got a new stereo within the last year and had Best Buy install it and am just noticing weird noises when the music is low. It's like a high humming noise.

Was wondering:
1. Whether to take it back to Best Buy and have them check out if it's an issue with wiring.
2. Whether to buy new speakers (going for less expensive) - What kind, if I can install them myself, if there's a way to find out compatibility or if there's universal.

Would like to know before I start buying things and whatnot, I'm not too experienced with cars, but can definitely go through instructions. Appreciate the input.

Crutchfield has a user guide, will tell you what fits where. Front doors are 6 1/2's, rears are 6X9's and dash tweeters are 1".
Try cleaning the battery posts really good, then charge up the battery. The "high humming noise" is alternator whine, it will vary in freq or pitch when you press the accelerator. Also check where they grounded the radio, do NOT use the ground in harness.

Thanks! After looking up a few things on your reply I was able to find out what to do and how to do it. I appreciate the help!