In the first episode, Basil says he is listening to Brahms. The scene appears in A Touch of Class, when Sybil complains that Basil has spent the morning hiding in the office instead of finishing his work.
Basil indignantly rejects her description of the music as racket: he identifies it as Brahms and jokes that he has been listening to Brahms's Third Racket. The line is a comic glimpse of his desire to appear cultured while running an increasingly chaotic hotel.
The episode is the show's pilot and first broadcast instalment, introducing Basil, Sybil, Polly, Manuel and the hotel's central tensions. Basil's taste for classical music is part of his attempt to raise the establishment's social standing, especially as he courts aristocratic guest Lord Melbury.
The common mix-up is with Beethoven or Mozart, both famous composers Basil might plausibly admire. However, the dialogue explicitly names Brahms; the joke also points specifically toward Brahms's Third Symphony.