Transistors as switch and amplifier to control LED strip
First, an NPN transistor isn't really a switch, it's a current amplifier.  If you put no current into the base, it allows no current through the collector.  If you put a small current into the base, it allows a larger current though the collector.  In digital circuits, it can be used as a switch by setting the base current to zero or quite a bit, and never anything in between.  But your circuit may well be operating for some of the time in the "linear" region where the transistor is acting as an amplifier.

Second, it takes about 0.6V to turn on an NPN transistor at all.  A silicon diode, as used in a bridge rectifier, also takes 0.6V.  And there are two of them in either path through the rectifier.  So nothing at all happens until the voltage exceeds about 1.8V.  The lower the volume, the less time the voltage will exceed 1.8V.