What hardware converts digital data to analog audio signal in Raspberry - past the codecs and the like, raw digital-to-analog conversion (be it using DAC, PWM, or any other means I'm not aware of)? Is there any device/interface/API to interface with that hardware directly in Linux, or would I need to hack my own kernel module? If the latter is the case - could you provide more reference on the related hardware? (references for SFRs that control the generated analog signal, or such?)
Audio signals are generated by the BCM2835s on chip DAC (connected via GPIOs 40 and 45), which has a driver for ALSA.
I doubt you would be able to access the hardware directly as audio samples seem to be processed in the VideoCore.
Of course you still have access to the ALSA APIs if those are good enough for your project.