Solo A Track
If you'd like to hear the audio on just a single track, without hearing other tracks, click the Solo button on the track header.  You can solo multiple tracks, as well.  For example, if you want to hear how the bass and guitar sound together without the drums, just solo the bass and drum tracks. 

You can also solo a track by choosing the Track menu followed by Solo (Ctrl+L) or by right clicking on a track and choosing Solo.