Unless something's changed recently that I'm not aware of, only the NHL and maybe some colleges stream for free. MLB and the NFL have charged for their streaming audio for at least 20 years, unless one pays for the TV package which includes the radio feeds.
In terms of streaming, Westwood One worked out an arrangement three years ago (I think) that allows its network stations to stream Westwood One's pro football games for free as well as allowing Internet listeners from outside of the individual teams' markets to hear the games on Westwood One's website without charge.
And, at least as far as pro football is concerned, @CTListener is absolutely right! All of the NFL games for each individual NFL team can be heard in each team's market on the radio with local play-by-play announcers and color commentators describing the games. Some of those local games have shown up online for free; others have not.