Hi,
It's magic.
In System Preferences > Sound > Input tab set the input device AND the volume.
It should not bounce the indicator bar to the far right (feedback and "motorboating" can occur at this point). If is is pinned to the far right it normally cuts the feed in the app your are using.
In Messages > Preferences > Video section is a drop down that should list the chosen device in System Preferences.
You may have to change this if there are two or more input choices.
Using two different sound options can introduce strange quirks.
Sometimes there is a delay from one end leading to the Input device "hearing" your speech coming back from the far end in an apparent echo and the doing the sonar type pinging of that until it effects only dogs. (making your ears bleed on the way of course)
Start with reducing the volume setting in the System Preferences after sorting out choices off device. (Set the System Preferences part first).
Then launch Messages and try a Video chat.
If they report you are quite you need to increase the System Preferences Input settings.
If they are quiet then they need to increase that setting at their end - try to stay away from using the Master Output volume as this tends to be the cause of being heard back over the connection and causing the feedback.
Unlike iTunes and some other apps there is no volume control in Messages (or iChat before it).
You can only choose the Input and output devices if there are Multiples (in fact the camera one becomes a drop down if there are more then one)
Basically it tends to be a delicate and magical balance between your settings, their settings and the quirks of the internet causing the odd delay.
In the Video Menu use the Connection Doctor items and using the Stats option during a Chat.
It should be stable. If it is not, go to the Preferences > Video section and select 500kbps as the speed.
This is fast enough for 4 way chats and can smooth out the variances in very fast connections.
If there are too many variances Messages will try to Buffer.
It drops Frames to do this and at some point this will effect the Audio.
The Buffering has not really changed since 1500kbps was about the max service you could get.
Unfortunately Apple have taken down the AIM test accounts that they used to run.
10:15 pm Friday; March 6, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
