Multimedia streaming when smartphones act as both clients and servers is difficult. Indeed, multimedia streaming protocols and associated data formats supported by today's smartphones are highly heterogeneous. At the same time, multimedia processing is resource consuming while smartphones are resource-constrained devices. To overcome this complexity, we present AmbiStream, a lightweight middleware layer solution, which enables applications that run on smartphones to easily handle multimedia streams. Contrarily to existing multimedia-oriented middleware that propose a complete stack for multimedia streaming, our solution leverages the available highly-optimized multimedia software stack of the smartphones' platforms and complements them with additional, yet resource-efficient, layers to enable interoperability. We introduce the challenges, present our approach and discuss the experimental results obtained when executing AmbiStream on both Android and iOS smartphones. Our results show that it is possible to perform adaptation at run time and still obtain streams with satisfactory quality.