negative base current in NPN transistor?
The led will not even light up until the signal amplitude is higher than the transistors Vbe (typically 0.65v for a bjt) unless it is DC biased by this amount. I would advise AC coupling the audio signal into the pre biased transistor, in order to get a better proportial relationship between signal amplitude and led brightness. Also reverse biasing the b-e junction is ok until you exceed the
V(br)EB at which point R2 will limit the Ieb (reverse base current) and the Ice current will be 0, Icb will be very small until V(br)CB is reached.