I am going to make a PTT project on Android. Could you tell me how deep Android supports Voice and Multimedia API (such as RTP,RTSP,VoIP) for developers?
MediaPlayer supports playing rtsp://.. URLs. Audio and Video are supported. Check media format support to see which codecs are supported.
MediaPlayer internally automatically handles RTSP and RTP, so there is not much you need to handle. OTOH it does not give any low-level control over this process.
About VoIP: Android only consumes RTSP/RTP streams, but does not create/publish them. So this is one-way only.
Android 2.3 (Gingerbread) is said to have support for SIP-based VoIP.