Hey, if you go to Settings -> Ringtones + Sounds, then select the individual things like New text or IM, new voicemail, new email. Click on them, and scroll to the top where there should be a "None" option awaiting you. Then they should just vibrate. Also, below those are some other options, like Reminders, Lock/Unlock, etc. There's even a checkbox for all other notifications. Ensure those checkboxes are cleared. You should then only get sound for incoming calls.
Of course, this is from WP7, it may be different in WP8, but it's worth a shot hey?