Best API for low-level audio in Windows?

It depends on what you are trying to do. The basic waveOut audio API is better for streaming audio. It lets you queue up several buffers and have them automatically played in succession. But if audio is playing and you want to change it, or add something to it, that's relatively hard.

DirectX audio is better for event based audio. You can have several things playing at the same time without having to do the mixing yourself. You can add or remove little pieces of audio easily - like playing a sound when the user pulls the trigger on their gun. But streaming (i.e. playing 1 buffer after another) is harder.

waveOut is designed to facilitate playing audio that is constant, like a .mp3 file. DirectX is designed for audio that is intermittent, like feedback in a game.

ASIO is like the worst of waveOut and DirectX in terms of difficulty of programming, and it's not that stable. Applications typically can't share the audio device. However, it gives you the lowest latency access to that audio hardware. ASIO also gives you a way to synchronize playback on multiple devices.

If you don't need to be able to change what is going to be played right before it is played, and you don't need to synchronize multiple devices, then you don't need ASIO.

At the time I asked this question, I wrote streaming code using the waveOut and waveIn APIs. Since then, I have discovered a useful library:

PortAudiohttp://www.portaudio.com/

PortAudio is free software with a commercial-friendly license. If you write your code to call PortAudio it should be able to work with waveOut devices but also with ASIO devices under Windows; it can be then recompiled for Linux and should work with ALSA devices; and it can then be recompiled for the Mac and should work with CoreAudio devices. I haven't tested the Mac part but my project is working great with Windows and Linux.

in addition to the options mentioned by John Knoeller, there is WASAPI which allows for much lower latencies than WaveOut, but unfortunately is only available from Windows Vista onwards.