^Thanks for the correction
In the late 90s, the company Big Finish got the license to create Doctor Who audio dramas-basically sound-only serials. The audio dramas feature cast members from the show, as well as several other notable British actors (David Tennant made his WHO debut in the audios, in fact). However they are mainly limited to the 80's incarnation of the show (Tom Baker refuses to do them, and his predecessors are all deceased, although Jon Pertwee did do similar projects in the 90s), with Peter Davison, Colin Baker, and Sylvestor McCoy, as well as Paul Mcgann who appeared briefly in the 1996 TV movie. Some people believe it's sort of a 'second chance' for actors like Baker and McCoy, whose eras were short-lived and frought with all sorts of behind-the-scenes drama, as well as Mcgann, who only got one TV movie. (Baker has probably benefitted the most from the audios in esteem among fans). In addition the audios have featured "What if" series (Unbound) and series dedicated to the Doctor's enemies the Daleks, Cybermen, companions Sarah Jane Smith, the planet Gallifrey, and an upcoming Davros series.
Another company, BBV, also publishes audios related to Doctor Who, but not referencing the Doctor himself, who is licensed to BF. These often feature cast from WHO but are not officially the Doc and co....they also feature monsters such as the Zygons, Wrynn, Sontarans and Krynoids, villains such as the Rani, and novel creations such as Faction Paradox and Iris Wildthyme.
BBV has also published some VHS and DVD series, such as the Stranger featuring Colin Baker and Nicola Bryant, the Auton trilogy and several others.