Not sure if the pearl interface is the same as mine, a Storm - I have an option on my main screen to fast-select ring volume (called Notification), which affects both calls and messages, and I can choose loud, soft, vibrate, silent, and also go into options for more detailed settings. You may have such an option that is easy to find, and maybe easy to set by mistake, without realising it.
Pearl manual at this location:
see p193, Notification Profiles.
There are other keys, eg # that affect notification sounds/vibrate. Download and search in the manual for 'Notification" to find all the places mentioned.
Even if you have a paper manual, I recommend keeping the electronic one since you can easily search for keywords and find all the places they are mentioned quickly.