From the Guide topic -

"Player Radio
Affects penalties, teamplay bonuses and more."

So this is the defensive player who has the mic from the coach so hey can call plays in from the sideline right? Forgive me I'm rather ignorant to this in reat life football, they never talk about the def side of the mic, always how the coach and the quarterback interact on the offensive side.

So, does this mean perhaps giving the radio to a player with high INT is advisable? What other factors should we consider when deciding who should have the radio? Perhaps Leadership?

Why does it affect penalties - in the sense that if the persion with the radio can get calls out and communicate to his defensive teamates, thus resulting in less penalies from confusion and the like?

What are "teamplay bonuses"?

I would definitely like to know a bit more about this as well.

Give player radio to defender you think best supports both run and pass scenarios and has a decently high intelligence to be able to make improved decisions and inform teammates. Basically helps with recognition of plays to defend passes better, make better tackles, etc.