You cannot
Media Playback developer documentation says (emphasis added)
You can play back the audio data only to the standard output device. Currently, that is the mobile device speaker or a Bluetooth headset. You cannot play sound files in the conversation audio during a call
Edit
In response to your edited question:
- I am not aware of Cyanogenmod permitting this- searching in their source code may give a clue
- To enable this, I presumed would require programming, so searched Stack Overflow and found this Why it is not possible to play an audio file on a voice call in android. This gives a couple of reasons, in answers as to why it is not possible - security and prioritization of audio
- It also suggests ways of achieving which are in programmers domain
