"But we're not supposed to talk anything besides football"

Al Michaels' quote recommends that the discussion should be restricted to football. This indicates that the speaker is in a scenario where football is the main subject of discussion, which any various other subjects should be avoided. It could be that the audio speaker is in a sporting activities bar, or at a football video game, or even in a team of close friends that are all passionate regarding the sporting activity. The quote could also be interpreted as a pointer to stay on subject as well as not obtain averted by other discussions. It could be a method of maintaining the conversation focused as well as productive. Whatever the context, the quote is a tip to remain on topic and not obtain distracted by various other conversations. It is a tip to remain focused on the task handy and also to not get averted by other topics.