I would call it a Perception DC of -10 ("Hear the sound of batte" = DC - 10) to hear the bard's performance. During combat, any creature trying to hear the performance would take an efective penalty of -5 to theirperception score ("Creature making the check is distracted" = +5 DC Modifier). From there, you modify the DC of perception by adding 1 per 10 feet of distance (again, essentially a penalty for the listener based on their distance from the source). So if you were 50 feet away, you would have to make a DC 0 perception check. For ease of use, I would assume that anyone within 50 feet can automatically make a DC 0 perception.
Beyond a (somewhat arbirtary) range of 50 feet, it will depend on what level each character has have invested into perception. You could either have them make checks each round, or for simplicity, have each character give you their perception modifier, assume they take a 10, and then tell them at what range from the party's bard they receive inspire courage bonuses.
Examples:
The party rogue has a perception bonus of +8.
He is in combat or otherwise distracted (-5 to his perception.)
His perception is thus +3 against and taking a 10 results in a +13.
The DC to hear the Bards Performance is - 10.
The rogue can thus hear the bard's performance from a distance of up to 230 feet.
The party fighter has a perception score of -1.
He is in combat or otherwise distracted (-5 to his perception.)
His perception is thus -6 against and taking a 10 results in a +4.
The DC to hear the Bards Performance is - 10.
The fighter can thus hear the bard's performance from a distance of up to 90 feet.
You can see that even in the worse case scenario, it's not that hard to hear the bard singing. This is the reason most people simply don't worry about it, unless they are doing battle over large distances.
I agree with MechE, the base should be a Perception DC of -10, and add 5 to it because the perceivers are distracted. How far away they can be and benefit depends on their perceptions, I'd just have them "take 10" for simplicity.
Or just say 100 ft for simplicity. 30 ft is way too short, you can easily see or hear someone trying to be heard/seen from more than 30 ft.
I also wouldn't mind if the bard could extend the range a person could see/hear by a perform check (or, to speed things up, again... what his result would be if he took 10). A bard with a very high performance modifier being able to extend his effects farther makes sense to me. Maybe say a check result of 20 or less is normal perception range for allies; for every point over 20, you get to extend the range an ally could normally hear/see it by 5 ft. Or something like that. How complex do you want it to be?