For audio everyone was using 3rd party tools like Miles Sound System etc., but even OpenAL launched around 2000 already as an OpenGL companion. Video had the same thing happen with everyone using Bink which launched around 1999.
In comparison using OpenGL was a lot nicer than anything before probably DirectX 9. At that time in DX you needed pages and pages of boilerplate code just to set up your window, nevermind to get anything done.
Advanced GPU features of the time were also an issue, OpenGL would add them as extensions you could load, but in DirectX you were stuck until the next release.