The format of .wav files containing PCM data (a stream of sample amplitudes) is reasonably well documented. You can find the information on the web.
.wav files can contain compressed data but you rarely encounter them.
PCM .wav files can contain audio at different sample rates and sample resolution. You probably want to pick just one type of .wav files that you support.
You might want to use a PC to extract the raw sample data and store in whatever memory you are using with your 8051. If you want to support anything other than linear PCM .wav files you could use one of the open source audio packages such as osalp that has converters for many audio formats.
Getting the samples from your memory device and playing them is hardware specific. If you have a general purpose DAC you will have to get do the timing with software counters or whatever internal timers your 8051 compatible chip has. You can also get DAC's intended for audio that can buffer samples and use a seperate clock.