Android - How can I disable the notification sounds while playing music?

First install Quick Settings: http://www.appbrain.com/app/quick-settings/com.bwx.bequick

With that under volume control you can control the following volumes seperately:

• ringer
• notification
• media (this would control the music)
• alarm
• voice
• system

On top of that you can install Quick Profiles

...which will let you create a sound profile with the settings you want and easily activate it with one click.

You don't need to download anything. Go into Settings and Volume, then turn off ring tone and notifications. Now, huzzah! Play music, still receive emails and messages but no irritating butt in of sounds!