BlackBerry Messenger is a instant messaging service available on almost all modern BlackBerry devices. The service allows BlackBerry users to quickly communicate with each other, regardless of which service provider their BlackBerry is registered with. BlackBerry Messenger uses a chirping sound as its default ringtone, so you can tell by the sound that a BBM message has come in, but you can change the ringtone to whatever tone you desire.
Select the “Profiles” icon, which looks like a speaker, on the home screen. Then select “Change Sounds and Alerts” from the menu that pops up.
Choose “Sounds for Select Profile” from the list of options. Then select “Instant Messages – Notifiers” from the next menu.