Consider that the power being sent to your speakers, is the AC coming from the wall. It gets adjusted by the transformer, rectified, filtered and passed through the output transistors.
Ideally, the filtration inside the components is such that no noise gets through. However, not always the case.
Any junk that makes its way through, gets amplified with everything else and mess up what is being amplified. One of the reasons systems sound better/worse on some days.
And, it will effect the sound as outlined by @steakster