It would not be theft, but it may be fraud: the knowing or intentional misrepresentation (lie) about a material fact (their ability to transfer the sound system to you), which you reasonably relied upon (no reason to doubt them). If it was fraud, that could provide grounds to either void the transaction (get motorcycle back) or get monetary compenation (e.g. value of sound system) from the other party. It may also be  breach of contract (even if the contract was simply an oral agreement to barter)--they were not able to provide you their part of the deal (a sound system which you would be able to own and keep). Therefore, you may wish to sue them for fraud and breach of contract.