FreeSWITCH is able to interface automatically with a lot of codecs and file/stream formats, and it can translate between them. This means that a CD-like source at 48 khz, 16 bit, stereo and wideband will be decoded, downsampled, truncated, mixed, and then re-encoded to be sent in a G711 call.
Keeping with the general FreeSWITCH philosophy of do not reinvent the wheel, audio files and streams are read and written using open source libraries: FreeSWITCH has a specific API for audio formats; anyone can write a wrapper for a new sound format library and that format will be available everywhere in FS that a sound format is used (the same applies to codecs and to stream formats; just implement their FreeSWITCH's API).
This ensures the most efficient and timely support for new file formats and codecs (Brian West released FreeSWITCH's support for BroadVoice codec 40 minutes after it was open sourced).