Dolby Digital, DTS, DVD-Audio's MLP, SACD's DSD, and CD's PCM are all examples of digital audio formats, but as DNicely1 points out they must all be converted to analog in order for a speaker (an analog device) to reproduce them. There are a few receivers and power amps that have begun using digital switching amplification (which has long been common on powered subwoofers), but we're still dealing with an analog signal - the "digital" term is somewhat deceptive.

Think of the signal path this way: digital from the source media (optical disc, satellite broadcast, cable TV broadcast, or over-the-air HDTV broadcast) to source device (DVD or CD player, satellite receiver, cable box, or HDTV tuner), digital from source device to processor (receiver or pre/pro), digital within processor for bass management and signal processing (Dolby Digital decoding, DTS decoding, or whatever surround processing), digital into D/A converter, then analog from output of D/A converter through volume control to amplification, analog amplified by amp and sent on to speakers, analog from speaker to ear. As long as our speakers are analog devices the signal has to become analog at some point before them, and as long as we're still using our eardrums to listen the signal will remain analog at our end.