Yes, the data that libsndfile returns is time-domain data: the audio samples from the input file. The function sf_readf_float
returns the samples in "frames", one frame being a set of samples from each channel at a given point in time. Also, sf_readf_float
scales the samples to be in the range [-1.0, 1.0] - see this note
for details.
Have a look at sndfile-tools for example code that reads sound files using libsndfile. The sndfile-mix-to-mono example should be a good starting point.