Another possibility.
I forget where the speaker grill is located, but check for gunk or some kind of crap inside. If you carry it around a lot in your pocket, a cloth lined bag, or something similar, lint will collect inside and effectively deaden the sound. Blowing it out may or may not work. I usually have to take apart the portables to get to the speakers to clean them. DO NOT USE compressed air to flush out the lint. If the cone is made of paper, you may just rip the cone rendering the speaker useless.
If the speaker grill is located where your hand is when you grip the DS, you'll get this thick goo inside which consists of sweat, skin oil and dead skin cells and other mystery junk. It's gross stuff. Luckily, it never reaches the paper cone, it just gets trapped in the speaker grill itself. Again DO NOT USE compressed air. Besides your risk of ripping the paper cone, you'll only push the goo further and onto the speaker. Not fun to clean off.
You don't exactly need to take apart the console to check for either, Check the speaker grill. Whatever is coating the cone will also coat the inside of the holes.