Doesn't need fixed. Was the same way on my HD7 and I never thought of a reason I would need an alarm to be quiet.
I hate this! I am a light sleeper and really like being able to control my alarm volume and not wake my wife. My WP7 HTC Trophy did allow the volume setting to control the alarm setting. Anyone know if this is a MS issue or a manufacturer issue. I have a Nokie 822 now.
I swear my HD7 used to respect the main phone volume. Generally I don't need to be woken up, just prompted to get up as I cannot see the clock in the dark. Additionally, I don't think my wife appreciates being startled out of her sleep.