Fix tuning in audio when periodic noise is not muted

[simondotm]

[EricLafortune]

The files data/bbcapple-palsms-3_2.vgm and data/bbcapple.vgm are identical. Did the original file get overwritten or did the transposition fail at some point?

I'm looking into this old issue for my more recent TI-99/4A version of the demo, which borrows the BBC chiptune (thanks for the amazing work!) The TI has a 16-bits shift register for its noise, while the BBC Micro has a 15-bits shift register, but the tuning comes out wrong on both systems.