The dependent clause follows the independent clause in this sentence, so no comma is needed.
Explanation:
When a dependent clause follows an independent clause, there is not need to include a comma as opposed to when the dependent clause is before the independent clause.
In this case the dependent clause being "because her brother’s stereo was turned up to full volume", follows after the independent clause which is, "Beatrice was unable to concentrate on her project". There is therefore no need to put a comma.
