Hello I have a set of Altec Lansing ADA215 speakers, and one of my speaker has very low sound level. This was not before I dont now what happend I hope you guys have an idea. Thanks B.Versloot

Try swapping the speakers around if the problem still persists either the speaker is faulty or
the speakers plug/outlet is usually at the back of the computer you will have to open up the computer to check the lead attached to the rear of the input plug make sure the connection is good
or you might have to clean the dust from your computer dust causes static which causes your computer to overheat and all sorts of problems
be sure to wear an anti static earth strap before touching anything inside your computer

have you tried this control panel sounds and audio,volume,advanced,speakers select the appropriate speakers from the drop down list or click start control panel administrive tools computer management device manager scroll to system devices + to expand,system speaker right click properties then select use this device (enable)
try cleaning your computer of dust using a fine air blower from a compressor making VERY sure that there is no MOISTURE in the air line specifically around the CPU central processing unit and your sound card dust causes static to build up in a computer causing it to overheat any many other problems
hope this helps