Out of the box the Xperia S supports a variety of audio codecs. Because of the open nature of smartphones however you can easily download replacement music players with different codec supports. The one that I would recommend using is UberMusic, but there are other competent apps to suit anyone's different needs. For the purpose of this review however I will be sticking to the default music player by Sony. FLAC lossless is not supported by the player, and while this may disappoint a few people, the vast majority will carry on fine with listening to high quality MP3 files.
Functionality wise, the Music Player is brilliant. It has a couple of features borrowed from their Walkman range, including SenseMe, the auto playlist generator that I raved about in my Walkman S630 review. Audio enhancements comes in the form of a five band equaliser, including a separate Clear Bass slider. Surround sound is also supported, which I found a gimmick and immediately switched off. Sadly it lacks the DSEE filters found on their Walkman devices, one that I found to be very effective when dealing with low bitrate music tracks. It does integrate nicely with Sony's own Music Unlimited store.
Much to my annoyance Sony has also included a setting to boost the loudness of the built-in speaker. As someone who frequently takes the bus, I can tell you nothing annoys us more than people playing their music out loud in public. The current track is shown in the pull down notifications area, but sadly lack any music playback controls. One the upside, the music playback controls do pop up when you enter the slide to unlock screen.
When it comes to sound quality, the Xperia S excels, at least when tested with my now standard Sennheiser IE 8 IEM. This is quite easily the best sounding smartphone I have heard in a while, and is at least on par with the iPhone 4S. Highs are a bit more pronounced, but apart from that everything sounds clean. Music signature is warm and deep. Nothing I have thrown at it suffered from any forms of distortions. Overall I think if you are looking for a smartphone that is capable of replacing your DAP, the Xperia S will suit you well. Having said that I wouldn't advice that you throw out your Walkmans or Cowons just yet as a DAP with a great DAC will still floor this. The Xperia S will drive most consumer-orientated headphones without any issue.