You get what you pay for - had mine for a week or two, working fine - now it keeps muting itself, or muting-unmuting intermittently for no reason, regardless of which USB port it's in. Not even worth returning. Will find a better one and sling this one soon. Shame, because sound quality was otherwise very good.
Update: Figured out that the press-able casing over the mute buttons had come lose and were pressing into the buttons inside, causing the mute/unmute problems. Managed to pull the shell open with surprising ease to refit the lose part - in fairness I have raised rating from 2 to 3 stars, but find the outer build quality poor compared to the decent sound quality it produces. If this was the same sound card housed in a solid case, with better buttons I would happily pay a few quid more.