Music master recordings for films are made in much the same way that classical recordings are made. Many are still mastered with Studer 24 track analog tape machines (the extra tracks are used only for emergency coverage if for instance an important flute solo needs to be brought up - the overall pickup is usually done with the "Decca Tree" configuration of 3 mics), and are usually mixed directly into a digital audio workstation at 24 bit resolution. The sound quality is every bit as good as what you would get on a DVD-A or SACD.

Dolby Digital and DTS are much like the mp-3 format where as much as 90% of the signal is "thrown away" because it is deemed "inaudible" by the codec. DTS does not discard as much as DD, but it is nonetheless a lossy format. This is how all film soundtracks are released in theaters and on DVD because of bit rate requirements which must make room for the video signal.

Most people don't notice the degredation since they have no way of hearing the original master recordings. In a direct comparison with the masters, all lossy formats are clearly deficient when played on good equipment in a controlled listening environment.