Earplugs give a bonus to save against sound based effects. Do they have any affect or prevent the positive effect of bard song for example?
As written no, BUT i do think most GM's would as a minimum give you a penalty on sound based perception checks. Bard Song however, has always been written oddly. There's no set distance, so I guess at gm descretion it also depends on what the bard is doing to perform. For example, if he is dancing, earplugs wont matter, if he's saying poetry, i'd probably say you need to be within 30'. Thats not a rule though, just some basic common sense added into a badly written item.
Made of waxed cotton or cork, earplugs give you a +2 circumstance bonus on saves against effects that require hearing, but also cause a –5 penalty on hearing-based Perception checks.
Since the bard's song only needs perception and not a perception check, there would not be any bad effect.
For example, a bard's somatic performance will not affect you, even if you are next to them, if you are blind. A bard's vocal performance that you cannot hear does not have an affect on you. I can't think of anything that would let a non-earplugged person get the benefit while an earplugged person in the same situation fails to get the benefit.
/cevah
Since the bard's song only needs perception and not a perception check, there would not be any bad effect.
<_<
What mechanic do we use to determine whether a person hears something?
If a bardic performance has audible components, the targets must be able to hear the bard for the performance to have any effect, and many such performances are language dependent (as noted in the description).
A 1st level bard can use his performance to inspire courage in his allies (including himself), bolstering them against fear and improving their combat abilities. To be affected, an ally must be able to perceive the bard’s performance. An affected ally receives a +1 morale bonus on saving throws against charm and fear effects and a +1 competence bonus on attack and weapon damage rolls. At 5th level, and every six bard levels thereafter, this bonus increases by +1, to a maximum of +4 at 17th level. Inspire courage is a mind-affecting ability. inspire courage can use audible or visual components. The bard must choose which component to use when starting his performance.
Hear the details of a conversation ... 0
Distance to the source, object, or creature ... +1/10 feet
While a check would only need to hit an easy DC, it is rarely asked for since you would have to be 50' away with earplugs to bring it up to a DC 10. Unless you dumped wisdom and did not put any ranks in Perception, a Take-10 will get that DC 10 easily. Thus GMs rarely even call for a check. Given the numerous people that tell you that Perception is the most important skill of the game, I think one can usually assume everyone has at least one rank.
If no check is called for (because the GM thinks it pointless, say), then no check is needed.
/cevah
Sure, but often when bardic performance comes up, you're in a position where you cannot take 10.