The high level layers encompass basically the winmm DLL exported APIs. It also provides the skeleton for the core functionality for multimedia playback and recording. Note that native WINMM and MMSYSTEM do not currently work under Wine and there is no plan to support them (it would require to also fully support VxD, which is not done yet).
winmm and mmsystem.dll16 in Wine can handle both 32- and 16-bit drivers (for low level and MCI drivers). Wine will handle all the conversions transparently for all the calls to winmm and mmsystem.dll16, as it knows what the driver interface is (32- or 16-bit) and it manages the information sent accordingly.
MCI drivers are seen as regular Wine modules, and can be loaded (with a correct load order between builtin, native), as any other DLL.
Multimedia timers are implemented with a dedicated thread, run in the context of the calling process, which should correctly mimic Windows behavior. The only drawback is that the thread will appear the calling process if it enumerates the running processes.
Wine also provide a dsound (DirectX) DLL with the proper COM implementation.