For films and music the audio data may have too much noise, but TV programmes with low background noise (news, documentary, interview) with available Closed Cap

For films and music the audio data may have too much noise, but TV programmes with low background noise (news, documentary, interview) with available Closed Captions (CC) are good training sources. CC transcripts are enforced by broadcasting regulators so they should be highly accurate. The big problem with using these sources is the huge vocabulary. Speech recognition works better for smaller vocabularies than bigger.