The MediaStartedEvent indicates that Communication Manager has started to transmit an RTP stream to the IP Address & Port that you provided in your Terminal Registration Request. It also provides other necessary data such as the IP Address and Port from which it is sending the RTP and the Codec and Encryption scheme it is using.
The RTP packets included header information and the media from the call.
You must ensure that something (your application or some other application) listens at the local port port and:
1. Receives the RTP data
2. If necessary, decrypts the media in the RTP data
3. If necessary, decodes the media
4. Dos something with the media. Most likely, buffer it and store it in a file.
5. Handles issues such as missing packets or out-of-order packets etc.
You can get more information on RTP in RFC3550
https://tools.ietf.org/html/rfc3550
You can either write some RTP stack software yourself or get some from a third party.
I should note that the DMCC Java SDK includes a very basic client media stack library which is used by the Softphone sample application to receive RTP and divert it to a speaker. That is what audio.start() does. However, this library is not suitable for a production environment.
Martin