For streaming analog (AAC, MP3, audiobooks, Internet music streams, etc.) from a Mac to an AirPort Express base station (AX), iTunes does most of the work:

• iTunes decompresses those file formats and creates what's essentially a raw, uncompressed audio stream.
• That stream is then compressed using Apple's Lossless Compression (ALAC) via a QuickTime codec, encrypted with 128-bit AES1, and then, streamed (via AirPlay) to the AX. ALAC is used over the UDP transport layer protocol to stream two audio channels.
• AirPlay uses the Remote Audio Output Protocol (ROAP) to establish the streaming connection type. By default, as you know it is set to 44100Hz @ 16 bits. at 44.1kHz. This cannot be changed as this is what the AX is expecting.
• The AX decrypts the stream, decodes the ALAC stream to an encoded digital audio (PCM) format at the same quality level of the original audio source format, and then, buffers it for up to 2 seconds before it outputs it to either:
  • An optical transceiver to convert the electrical signal to an optical one before sending it to the innermost part of the AX's audio port, OR
  • As input to the built-it Texas Instruments DAC to be converted to an analog audio format before sending it to the outermost part of the AX's audio port.

https://discussions.apple.com/thread/8509040?answerId=33786684022#33786684022