Why do my harmonica notes sound bad? (+jaw, mouth, tongue position)

It's very unlikely that your 2-draw or 3-draw, and/or upper register reeds (8, 9, or 10) are faulty. The odds are very high that your harmonica is working fine. All you need to do is make a small adjustment in your technique.

Here are your three main goals:

1) Drop the jaw (so your teeth are as far apart as possible

2) Open wide on the inside of your mouth by simultaneously yawning and saying "ahhh" (like the doctor says, "open wide and say ahhh")

3) Maintain the Deep Relaxed Mouth Position so you can still play a single note (Often times when students drop the jaw they loose their single note. You need to learn to be able to play a single note while your jaw is dropped. So ultimately, you will find a mouth position that enables you to have TOTAL FREEDOM OF MOVEMENT OF THE JAW while still easily being able to play a single note. Remember! The key is to angle the harmonica into the lower lip, so that lower lip obstructs the undesired holes.

So what are you doing that's not helping? I'll take a educated guess. The problem could be that you're teeth are too close together and you're probably lifting the back of your tongue and it's creating a slight jet stream with pulls the reeds into a bent position. This is a very normal problem and it happens to over 90% students in the beginning.

I'm going to reiterate the above three points in hopes of being more clear:

Try adjusting the position of your tongue. Experiment with moving it into a variety of different positions to see if this helps. My suspicion is that you are raising the back of the tongue and you need to open your mouth wider. The reason I am bring attention to "the back of the tongue" is because students often think they are dropping the back of their tongue but they are only dropping the front of their tongue.

Try OPENING WIDE, DROP THE JAW and say "ahh" or "ohh".

In order to drop the back of the tongue, it is very helpful to DROP THE JAW. Open as wide as you can while still allowing the lips to maintain their seal on harmonica. If when you drop your jaw you can no longer play a single note, check to make sure you are still angling the harmonica into your lower lip or go back and make sure you are properly using The Deep Relaxed Mouth Position. 

Dropping the jaw is the key. 

I would also recommend that you play at a moderate volume. A lot of times students suck (and blow) too hard when they are first learning and this only makes the problem worse. Try playing more softly and see if this helps.

Go here for more info on the Deep Relaxed Mouth Position.