There are a few apps out there that let you control the volume for Alarm, Media, Alerts, Ringer, and Call Voice individually. The two I'm familiar with are AudioManager, which has a nice widget with slider bars to adjust all of the above, plus a quick way to set alerts, ringer, and system volume to vibrate, and Volume Toggle which allows you to set up profiles with different volume settings. For example, I have a Sleep profile that has everything silenced except the alarm volume. Try out both and find out which one you prefer, but both should accomplish what you're trying to do.