ensure api complience with audio for multimodal messages

[rachwalk]

This task contains 3 steps:

1. delete the if self.audios not in [None, []]: check that was blocking audio support
   

2. add support for base64 encoded audio files in the __init__ method
   

3. create audio content entries similar to how images are handled using appropriate mime type for audio (e.g "audio/wav") 
   

A PR can be made with these 3 features.

[mdimado]

Thank you so much for the sub tasks. please assign