How to add an AudioSource based class to a mixer

Hello there.
I’ve created a class for managing the audio of my synth:

class midiSource : public juce::AudioSource

I would like to add it to my mixer. However, the MixerAudioSource class accepts only AudioSource as input. How can I pass midiSource as an input source to my mixer?
(e.g.: mixer.addInputSource(mySource, false);)