If you want your application to use the default port, you don't have to call the IDirectMusic::CreatePort method before adding the port to the performance. You can instead pass NULL to IDirectMusicPerformance::AddPort.
In the current version of DirectMusic, the Microsoft Software Synthesizer is the default port. However, you should not assume that the default port has the features you need or that it will always be the Microsoft Software Synthesizer. It is good practice to obtain the default port by a call to IDirectMusic::GetDefaultPort, then check its capabilities by using the IDirectMusicPort::GetCaps method. If the port does not meet the needs of your application, you can use the IDirectMusic::EnumPort method to find the Microsoft Software Synthesizer or another port.