**You cannot**

[Media Playback](http://developer.android.com/guide/topics/media/mediaplayer.html) 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](http://stackoverflow.com/questions/27602114/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](http://stackoverflow.com/a/27918838/5585644) and [prioritization of audio](http://stackoverflow.com/a/27981383/5585644)**
* It also suggests ways of  achieving which are in programmers domain