The theory covers speech as well as music or other background sounds.
There have been many studies on this theory and it has been consistently proven that unrelated or irrelevant background sounds inhibits one's ability to perform well at serial recall.
[1] It is possible that the visual stimuli (the list of items) is held in working memory as a phonological code.
[2] If that is the case, the irrelevant speech could interfere with the articulatory rehearsal process, degrading the information in the phonological store.
This would result in degraded performance on trials where irrelevant speech is presented.