Start by trying the sound on something not related to the internet. (e.g. assuming that you're using a PC or laptop with a CD drive, try playing a CD).
If the sound's still intermittent then it's probably a physical problem, such as a loose connection, a dodgy volume control on your speakers or something similar. If the sound's OK with a CD (or when playing an MP3 file) then it might be a browser problem. (Try a different browser).
However it really would help to know what type of device you're using. (There's not much point in people here suggesting ways to fix a PC problem if you're using an iPad!)