That's not available right now is the OS. Hopefully it will be with the update that is supposed to be coming out this month and next. I'm looking for the same thing. Same for vibrate on SMS/Email, but ring only on phone.
had a similar volume issue on my HTC 7 Mozart. i amplified the volume on my favorite ringtone, alarm and alerts and used the Chevron tools to sideload back to the phone. huge difference now. not my preferred method to address the problem, but it will suffice for now.