I have a Blackberry 9700. Since 3 days ago, when I make call or when I receiving a call, I can't hear anything UNLESS I activate the speakerphone. What could be the issue?

Otherwise, when playing music the sound is okay, the alarm tones, the ring tones, are audible.

The first thing to tyy is a battery pull to reload the OS clean
the other could be either the OS is corrupted or an App is causing this. you say it works ok when listening to music so i doubt it is something caused by damage. you need to delete any apps you added just before this happened or upgrade the OS if the battery does not work.