This paper proposes a combined network and application layer access network decision model for multimedia applications in heterogeneous networking environments. It builds on previous work of a network layer based metric used in combination with multihomed Mobile IP and introduces a mechanism for applications to interact with the mobility management system in the mobile node. This way, applications executing in the mobile node can decide either to take access network decisions on their own or to let the network layer handle mobility management tasks automatically based on default decision criterion decided by the end-user.An extended architecture based on previous work and the upcoming IEEE 802.21 standard for media-independent handover services is presented. The control plane, named "Mobile Mediator Control Function", offers a set of events and commands through an additional service access point. Results from a scenario with a Voice over IP application running in the proposed environment simulated in OPNET Modeler are presented.