libsydney and portaudio are wrappers like Phonon. In the same category there are also libao, SDL-audio and others.
What I miss most with PulseAudio is AC3/DTS/AAC passthrough. I hope that will be implemented soon. It's not too uncommon to have the computer connected to a receiver via SPDIF nowadays, and having it go through the AC3 -> decode -> PA -> ALSA AC3-encoder -> receiver is a lot worse CPU wise than just AC3 -> receiver when the system is already struggling with being able to decode the 1080p video stream in realtime.