In the new speakers, the sound is either quieter or louder. What could be the problem?

Question

Sound cards

I bought Sven BF-01 speakers and after playing music I noticed that the sound suddenly becomes quieter, and then returns to its original volume again. What could be the problem and is it worth going to the store and changing to others? On a windows 8 64 computer, the computer is hp envy m6 1260er.