Q. Can I do other things in the background whilst playing mta.
A. No, because alt-tabbing is disabled. There are hacks which allow you to play in windowed mode, but they are disallowed by the EULA and therefore it is against the LAW to use them
Q. Are the sirens synced?
A. Yes, like you say the primary siren is synced, the secondary is not.
Q. How do I go faster on bikes.
A. Tap your lean forward key, or if you use a controller which has proportionality, hold the left analogue stick forward enough so that the character leans down into the bike. Maximum speed will be achieved when you lean the furthest forward possible (not when the character stands up).
Q. Why doesnt the dumpers tray move, forklifts fork lift, tow trucks tow lower.
A. The dumpers tray is static, as is the tow trucks hook, but the forklift's forks can be raised and lowered. I'm not sure which keys do it, but on most controllers it's X-axis movement on the right analogue stick.