Without knowing what speakers you have I will try to give you some ideas. First make sure that your soundcard is set correctly. If it is set for surround sound some sounds will not be heard as there is no speaker connected.
The speakers themselves may have a problem such as a blown midrange or tweeter. When this happens in one of a pair it can be hard to pin down so download some 20-20k pink noise as a test http://www.dogstar.dantimax.dk/testwavs/
and listen to each speaker with this hissing type of sound. Both speakers should sound the same if they are working properly.