"There is a speaker in the bathroom, but not a mirror."
Translation:화장실에 스피커가 있지만 거울은 없다.
은 is often used when putting two contrasting statements next to each other. This way, it can be placed either before/after other endings or just replace them (the way of treatment depends on the ending itself). For example: 어제 집에 가고 오늘 학교는 간다 - Yesterday I went home and today I'm going to school (~에 is replaced with ~는). 그 책은 읽지 밀고 이 책은 읽으십시오. - Don't read that book, read this book (~를 replaced with ~는; note, that 는 is used twice, which is perfectly fine).
The sole purpose of it is to exaggerate the contrast between the two parts. Because of that, the first sentence can be also translated as "Yesterday I went home but today I'm going to school".