Despite your post being moved to this call sound quality catch-all thread, your problem sounds like it might be more likely to be the problem described toward the end of this thread, where no sound goes from the phone to the other side, but the other direction works. You may be able to work around this by switching the speakerphone on, then off. If that is the problem, doing so may cause the sound to work until the phone is rebooted.
This seems to happen more often in the US than Europe, but does happen there as well.