When attaching a gun/turret/cannon to a vehicle, changing direction of the gun/turret/cannon does not sync. For other players, it is always shooting straight ahead, even if it's facing behind the player who is driving and attached it. As far as I know, the original guns/turrets/cannons on their respective originating vehicles are synced fine. The shooting on all of them works as well, it's just the direction the attached ones are aimed in that causes the issue.

Also, if you attach the tank cannon to a car, it will often "attach" itself to a car window, and if you turn the tank cannon to another direction to aim it, the window will turn with it, although this bug is not synced, so others do not see it, likely to the above bug in this post. It's likely a GTA engine bug since it was never made to do this, but figured I should throw it in there just in case anyway.

As the title says, it would be pretty cool to have a function called Vehicle.Alarm to set/get the vehicle's alarm status, and to enable/disable it under different conditions, such as attempting to steal or ramming the vehicle hard enough, just like in Single-Player.

The version you are using: 0.1.0.9
What you/the other player were doing when you noticed the bug: Driving near the club by El Burro's Payphones in the Diablo Territory of Portland, (can't remember the name of the place off-hand).
Is it reproducible? Yes.
What you would expect to happen if the bug didn't occur: Normal club sounds to play.
A brief description of the bug: When near the club building featured in a singleplayer mission, located in the same area as El Burro's Payphones, instead of club music playing, a very creepy, non-understandable argument between two male voices can be heard repeating randomly. The building on the opposite side of this area plays the music just fine. See the image for view of the building itself which has the bug:

http://i208.photobucket.com/albums/bb187/ms-sugard-x/lusoundbugbuilding.jpg

If you go close to the building, (much closer than I am in the image), and wait for a moment with your sound turned up, you can hear the voices arguing. This does not exist in singleplayer, and instead should be the same club music playing as the building opposite of it.