Brahms Em is one of the absolute standard works for cello. Definitely a suitable choice for an audition (in fact I reckon it's mandatory for many jobs). Chances are you will have accompaniment... bring the score to the audition at any rate, the conductor may play the piano part or at least read it and hear the proper thing in their head.
(If you've not actually rehearsed the piece with a pianist, then it may not be a good idea to let them accompany you, could throw you off more than help...)
As the others have said, in an audition for an orchestral position, it's quite as important that you're able to sight-read decently well, i.e. you'll probably be handed at least one simple accompaniment passage from some symphony and expected to get it right on first reading. Likely, you'll also be given some technically demanding passage that may require nasty thumb position or something – just don't panic if that happens, they'll want to know your limits, but it's natural that most players won't get it anywhere near perfect at first sight.