Chic01taliano: Hi guys, i'm looking into the issue. The thing that i want to clear up is that it has nothing to do with the audio output: not even the subtitles are displayed.
Some bugs were introduced with the latest patch by the developers. If putting the cell phone away doesn't initiate the mission (you can tell by looking at your map), then you may have to restart your save.
Keep in mind that radio towers all say the same thing; essentially, "go kill this person", and it will show where he's at on the map.
Other missions may be a little more tricky as they sometimes have you doing more specific things but they can still be done. And if it's a main story mission your buddy may call to give you an alternative way to complete it, which gives you another chance to get clued in on what's happening (that is, if it doesn't bug out twice in a row).