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It basically depends on how the particular musical performance is perceived by the listener. Cognitive process of listening seems to be comprise several layers, which follows a bottom-up direction.

First step is to decode relevant signal(s), among a complex package of sound. This is where the irrelevant noise is eliminated. Can music be eliminated in this level? Highly unlikely, but still possible. I do not know of a particular experiment but when the music is being played in a far destination, or with a low volume, or if the participant is highly concentrated to the task; then it may be eliminated in this step. But the key point in this step is that the term "noise" refers to aperiodic background sounds. Therefore, my first impression is that music, being periodical, must decrease the task performance. Cutler and Clifton (1999) gives an overview on the entire listening process. Second step is the grouping of different sound sources. There is also modelling studies that aims to explain this phenomenon (Bregman, 1990). Steps in listening continues further, but those steps are beyond the scope of this question.

But there are other studies also. Ylias and Heaven (2003) showed that the background noise negatively effects reading comprehension. So far so good. Cassidy and MacDonald (2007) showed that task performance on silence is greater than in low arousal music, and that is greater than noise, and that is even greater than high arousal music conditions. This is interesting, because it now introduces the affective state of the listener into the equation, which makes it a lot more difficult to handle. Another result is that the effect of the noise here is comparable to the effect of background music. But we have to note that the details of the noise in this experiment is not given in detail, only commented as "the everyday noise". It would be more conclusive if we just know whether it is the background sound of a television (periodic) or a traffic noise (aperiodic).

Combining these references, I cannot easily conclude that music is taken as a "noise". It seems that music reduces the task performance, by negatively affecting a later step in the listening process.

Ending note: There are several semiformal-informal studies on the web also. They study directly the "work/office performance", therefore I must say that they lack a little bit of a controlled environment. In such environments we can even confidently say that music improves our performance in particular situations. But what we miss is that office environments comprise several unhandled parameters that makes it hard for scientific experimental setup (i.e. listening music may improve the performance if your office mates chit chat next to you).

Ending note 2: I was interested in this topic a time ago. So I welcome more recent references or comments.