Typically, "luisteren" is non-transitive. The verb doesn't take an object. "Ik ben aan het luisteren." "Ik luister." "beluisteren" is transitive. It takes an object. "Ik ben een cd aan het beluisteren." "Je kunt je voicemail beluisteren door te bellen naar 3333." ("je voicemail afluisteren" is also very common, although "afluisteren" typically means eavesdropping in other contexts). "beluisteren" is an almost exact synonym for the construction "luisteren naar" (listen to). I feel that "beluisteren" sounds more somewhat formal and somewhat stiff (which can be on purpose). However it has the advantage of being shorter dan "luisteren naar", which can be practical in technical writing. The same pattern is found with "kijken", "bekijken", "kijken naar". However I don't experience "bekijken" as formal or stiff. I could easily say on the phone: "Ik ben het nu aan het bekijken". I could just as easily say "Ik ben er nu naar aan het kijken." though. To add some complexity, in particular cases, both "kijken" en "luisteren" can be transitive. "Ik ben het journaal aan het kijken" (I'm watching the news (i.e. on television)). " "Ik ben televisie aan het kijken". "Ik ben radio aan het luisteren." , "Ik ben de nieuwe cd van Adele aan het luisteren." I don't think there's an English equivalent for this use of "luisteren", think of it as "watching", but auditory. This use of the verbs doesn't always sound right, and I don't know the exact rules for it. I think it works really well for just describing what you're doing at the moment. "beluisteren" en "bekijken" are always transitive. In a relatively casual story, I'd use "luisteren naar". "Toen hij in een muziekwinkel naar een cd stond the luisteren." Like Alex says, the given sentence is not complete. It needs a regular clause either at the front or the end of the subordinate clause.

beluisteren ~ to listen to ; luisteren ~ to listen. In your example : a cd - to listen (to). A dutch person will probably give you a more detailed answer on this. Also that phrase is missing a verb.