Imagine a combat with a spellcaster at the top of initiative. He casts Ear-Piercing Scream at somebody. They fail their save, so they take some damage and a Dazed for a round. Next is another spellcaster, who casts Command at somebody else, who also fails their save. On their turn, they will be forced to flee at top speed.
Next is me, a bard. I begin a Countersong with a very high Perform (Sing) result. Both the target of the Scream and the target of the Command use this result as their new saving throw, and they succeed.
What happens? Is the target of Command still obligated to flee? Is the target of the Scream still Dazed? Does the damage done by the Scream go down by half?
Ear-piercing scream is an instantaneous effect (per spell description, even though it has a non-instantaneous rider effect) and therefore Countersong explicitly doesn't work against it.
Command - expect table variation. I would rule that the "sound" part of the attack (I.e. The caster commanding, "Flee!") has already passed by the time you get Countersong up, so I would rule that it does nothing, but I could see a generous GM giving another save. I could also see a GM ruling that Command is simply a spell with a verbal component, not a sound-based attack at all, and therefore Countersong does nothing against it.
Personally, I would let you use Countersong as a defense against either of these spells *if it was already up* but would rule that it does nothing in the situation you've described.
I've found it most useful against harpies and other similar types of attacks. Very situational, but handy when you need it.
Also, it's worth noting that it's completely independent of your Bard level, so this can be very handy for someone who just has a 1 level Bard dip. My Magus has 1 level of Bard (for Dervish Dance) but has maxed out Perform Dance and Sing for Countersong and Distraction (distraction has never come up yet - at almost 12 - but Countersong has saved the party 2 or 3 times).
Hmm..... I figured the it might not work against Scream, but I expected it would work against the likes of command. If there needs to be an ongoing sound to counter, how could it possibly be of any use? Much less an ongoing "language-dependent" effect?
Could I get a second opinion?
My experience is that it's not that useful (and Distraction even less so), but that when it's been useful it's SUPER useful. I've used it to basically nullify a gaggle of harpies a couple of times. And there was another time when we ran into some kind of singing plants. But my advice is to know the rules for each of them enough to be able to explain to your GM (many are not familiar with them because they come up so infrequently) and then expect table variation.
But of course that's just a second opinion from the same person so hopefully someone else will chime in and explain that I've got it all wrong (I would love to find out that they are more useful than I'm describing, because I've got two PCs with 1 level Bard dips).
Lemme see if I'm reading this correctly:
1. Spellcaster Alice casts ear-piercing scream at Victim Bob. This occurs instantaneously, so the damage is done and Bob is dazed for a round while they recover. If Bob were being dazed because the scream continued for a round, that would be a different story, but in this case Bob is dazed as an aftereffect.
2. Spellcaster Charlie casts command. This occurs instantaneously, so the damage is done and Bob is locked in to the effect of the spell. If command required the caster to continue to repeat the command for the rest of the round, that would be a different story.
3. Bard Delilah begins a countersong. However, there are no current sonic or language-dependent magical effects occurring at the moment, so nothing happens yet.
Let's continue!
4. Two rounds elapse. Delilah is still countersinging.
5. Alice casts ear-piercing scream at Bob again. This time Bob gets to use Delilah's Perform check, if it's higher, possibly preventing Bob from being dazed.
6. Charlie casts command on Bob again. This time Bob gets to use Delilah's Perform check, if it's higher, possibly preventing Bob from having to drop his pants. (His spare pants. That he was holding in his hands. It was laundry day.)
Bottom line: Countersong only messes something up if there's something audible to mess up. It's interference, not medicine.