You sure can!
You'll see on individual track pages that some songs will already have a choice of keys available. These are either keys that have been previously requested, or that we thought might be useful! You'll find these listed as 'Original' key (i.e. the same as on the original cast recording) and options marked with how many semitones away it is (e.g. '-4' is 4 semitones down, '+4' is four semitones up). We try to be musically sensible about this because composers do choose their keys carefully - it might be impossible for most mere mortals to sing it any higher/lower, and sometimes the arrangement will just sound odd in a different key, so it's not always a good idea to transpose.
However, if you'd like the track in a key which isn't there, get in touch and we can do this for you for £15+VAT. We arrive at this cost because we completely re-record and re-sample each new key, rather than just pitch-shifting the file, as we want to provide top-quality, professional standard backing tracks. Simply pitch-shifting a track will result in quite a loss of quality and you'll end up with some weird, warpy-sounding artefacts. (If you're not that fussed about that, you can always pitch shift it yourself in some free audio software!) Just drop us an email with the song title, musical and the key you want and we'll sort you out!