Based on code originally contributed by Marc-André Lureau we now support Flat Volumes. The idea behind flat volumes has been inspired by how Windows Vista handles volume control: instead of maintaining one volume control per application stream plus one device volume we instead fix the device volume automatically to the "loudest" application stream volume. Sounds confusing? Actually it's right the contrary, it feels pretty natural and easy to use and brings us a big step forward to reduce a bit the number of volume sliders in the entire audio pipeline from the application to what you hear.
The flat volumes logic only applies to devices where we know the actual multiplication factor of the hardware volume slider. That's most devices supported by the ALSA kernel drivers except for a few older devices and some cheap USB hardware that exports invalid dB information.
On-the-fly Reconfiguration of Devices (aka "S/PDIF Support")
PulseAudio will now automatically probe all possible combinations of configurations how to use your sound card for playback and capturing and then allow on-the-fly switching of the configuration. What does that mean? Basically you may now switch beetween "Analog Stereo", "Digital S/PDIF Stereo", "Analog Surround 5.1" (... and so on) on-the-fly without having to reconfigure PA on the configuration file level or even having to stop your streams. This fixes a couple of issues PA had previously, including proper SPDIF support, and per-device configuration of the channel map of devices.
Unfortunately there is no UI for this yet, and hence you need to use pactl/pacmd on the command line to switch between the profiles. Typing list-cards in pacmd will tell you which profiles your card supports.
In a later PA version this functionality will be extended to also allow input connector switching (i.e. microphone vs. line-in) and output connector switching (i.e. internal speakers vs. line-out) on-the-fly.
Native support for 24bit samples
PA now supports 24bit packed samples as well as 24bit stored in the LSBs of 32bit integers natively. Previously these formats were always converted into 32bit MSB samples.
Airport Express Support
Colin Guthrie contributed native Airport Express support. This will make the RAOP audio output of ApEx routers appear like local sound devices (unfortunately sound devices with a very long latency), i.e. any application connecting to PulseAudio can output audio to ApEx devices in a similar way to how iTunes can do it on MacOSX.
Before you ask: it is unlikely that we will ever make PulseAudio be able to act as an ApEx compatible device that takes connections from iTunes (i.e. becoming a RAOP server instead of just an RAOP client). Apple has an unfriendly attitude of dongling their devices to their applications: normally iTunes has to cryptographically authenticate itself to the device and the device to iTunes. iTunes' key has been recovered by the infamous Jon Lech Johansen, but the device key is still unknown. Without that key it is not realistically possible to disguise PA as an ApEx.
There have been some extensive changes to natively support Bluetooth audio devices well by directly accessing BlueZ. This code was originally contributed by the GSoC student João Paulo Rechi Vita. Initially, 0.9.15 was intended to become the version were BT audio just works. Unfortunately the kernel is not really up to that yet, and I am not sure everything will be in place so that 0.9.15 will ship with well working BT support.
There have been a lot of internal changes and API additions. Most of these however are not visible to the user.