Slightly improved introduction animation and one fix

The animation after each game start now includes falling girders (on all Spectrum types). But for now, all girders fall at once and the monkey doesn't jump to the left and grunt (it only does so after loading the game or in the repeating introduction on computers with extended memory). I need to free up more memory for better animation after the game starts.

When AY sound wasn't active (beeper sound was active) and the player turned on or off the background music and then turned on AY sound, the AY sound was broken. It is fixed now.