GarageBand monitor outputs sounds much different from actual sound.

Are you using headphones or speakers to monitor the sound? What kind of mic are you using. Is your voice recorded on a dry track? Do you have any effects on? This doesn't have anything to do with RAM.

Also, what preset did you use for the vocal track? If you're a guy but chose a female rock vocal track setting, it would sound higher. Make sure "No Effects" are chosen from the menu on the right.

Do yo mean by monitor, what you hear WHEN you are singing? Everybody's voice sounds bigger and fuller to their own ears WHILE they're singing because all your sinus cavities act as resonating chambers that are all connected to your ears.

You won't be able to reproduce THAT sound.

If you are using headphones, then it applies a little less, but you still get the vibrations from the resonating chambers.

try recording someone else's voice and see if you have the same issue.

"M-Audio USB-powered digitizer."

What model?

When you play back other tracks, do they also sound higher pitched?

You have run into the most common issue with recording your own voice... Welcome to the club. Choosing the right mic for your voice is a big part of getting a good recording.

Recorded vocals sound best through a large diaphragm condenser mic with the hottest possible input signal without clipping. Record with no effects and keep the EQ flat. This will allow you the most latitude for correction in the mix later.

A trick to get deeper resonance on a male vocal goes like this... Use a second LDC on a second track placed closer to the body and pointed at the chest. This picks up more of the low-frequency rumble of the body cavity and can be mixed with the primary vocal mic in post production. A variant of this is to use the second mic placed behind the singer pointing at the back of the chest. This gives a similar result but watch out for phasing issues...you can really kill a recording if things get out of phase...

I like that body micing tip!

"Recorded vocals sound best through a large diaphragm condenser mic with the hottest possible input signal without clipping. Record with no effects and keep the EQ flat. This will allow you the most latitude for correction in the mix later."

Very true. Except in GB you don't have the option of recording the effects (EQ). Those get applied after the recording. Most other programs DO have that option though, so it is good to know...

Sounds to me like your playback from your monitors is also being recorded on top of your voice track. You may want to monitor playback through headsets and see if that helps.

Are you monitoring from your Mac or your M-Audio? I'm not sure what the digitizer is doing but does it have different settings for bit rate/clock speed.