Can I disable e-mail notification sounds streamed to a bluetooth hands-free car radio
You could use an app like Tasker. Next to Location and Time, Tasker can also act on certain system events. I use this e.g. to "switch to night mode" when I place my Droid2 onto its docking station, turning it (amongst others) into "silent" mode. Similarly, you can have it react on "Bluetooth connected", and have all volumes adjusted to your convenience -- e.g. notification volume to 0, but media volume to 6 (for some music). Many more possibilities wait there...