Is it permissible to state an untrue reason for leaving an accommodation to avoid smoking and listening to music, out of consideration for the feelings of flatmates, and would that be considered a prohibited lie?

It is not permissible to sit with a smoker while he is smoking, except out of a need or for a legitimate Sharia-compliant interest, along with censuring him. If you fear harm to your body or temptation (fitna) in your religion from living with smokers, then it may become obligatory for you to separate from them. If you censure them and they do not respond, then it is preferable to leave living with them unless you are compelled. You may inform them of the real reason or allude to it without lying.