I am sorry to report that I also have experienced the Low Volume issue or maybe the fading in/out ( hard to hear when volume is so low)
i ONLY use 3G and Skype ( over 3G), talking on the handset.

talking on a bluetooth Stereo headset Nokia BH-503 makes it even worse.
it is hard to accept not hearing other people if you are making biz calls...make you look stupid