First, iI think that, despite his best intentions, your boss is a bit of an idiot for posting cash through the letterbox knowing about the possibility that you live with other people.
In terms of your options, your housemate said she would be back and she would check her room for the envelope. I think you need to wait until she gets back and does that. it may be the case that she has never seen the envelope and her absence from the house, and failure to reply to your texts, has nothing to do with your money and has more to do with personal problems she is experiencing. It may also be that she has in fact taken your money and is now working to try and have it replaced. Indeed, it is equally possible that the other housemate has taken the money and is attempting to deflect blame away from himself.
You cannot possibly know what did in fact happen and it may even be that your boss never sent you the money at all - or posted it through the wrong letterbox. As people have already said, it is best to talk to your housemates about it rationally and listen to what they have to say. You will probably know if they are lying.
thanks for comment. i wouldn't call him an idiot, it was in an envelope addressed to me, you should be able to do that but we won't be doing it again, first and last. i checked all that and he did deliver to right address my housemate confirmed this.
just have to wait and see how it pans out after a meeting. i heard letter box go this evening and she had delivered her rent but didn't attempt to come in and check her room for the envelope.
i've the intellectual confidence to appear stupid sometimes…