Volley archery isn't a terrible concept... I think it's actually a lot more useful than the super-long range archery, which is really really cool when you pull it off, but won't necessarily be feasible every session. It's a nice trick to pull out of your hat if you can, but I hate to see builds more or less constructed around the idea.
For volley archery to be successful, yes, you want to be able to fire as many shots in a single round as you can, but as important is adding bonus damage to each shot. You want to fit on as many of the following forms of bonus damage as you can:.
-Strength bonus from composite bows, or dex bonus from crossbows with Crossbow Sniper.
-Knowledge Devotion (no volley archer should be without Knowledge Devotion).
-Sneak attack/sudden strike. Requires a reliable way of getting sneak attack in, and requires being within 30'/60', depending on the build.
-Skirmish damage. More useful than the above because of its easier trigger, and especially good for mounted archers, who can move and take full attacks. Still requires being within 30'/60'.
-Favored enemy.
-Magic bonuses. The best way to really do this is to focus exclusively on enchantments that add bonus damage to your weapon, and then use GMW or Metaphysical Weapon.
-Spells and/or powers. Too many to list.
-Weapon specialization and the associated feat tree. Generally not worth it, because the levels required to invest in it preclude you from taking on most of the other options.
For getting as many attacks as possible, you want to look at ways to increase the number of "attack" actions you get, and ways to increase your number of attacks within those actions.
-As has been mentioned multiple times, polymorphing/metamorphosing into an Arrow Demon and using Greater Manyshot. If you can reliably do this, it's hard to find anything much better.
-Spells/items that give you extra attack actions. Boots of Speed and/or straight-up casting haste is pretty much a must.
-Class abilities such as a barbarian's Whirling Frenzy (UA variant).
-Feats that increase your number of attacks. You are either looking at making full attacks, or manyshotting. In general, full attacks are preferable, except in some specific scenarios (Swift Hunters with no mount, the above-referenced Arrow Demon setup, etc).
-Two-Weapon Fighting. Only really applicable with crossbows, and you need either Quick Draw and a whole lot of hand crossbows or some method of reloading during your action. That being said, the TWF chain can get you an awful lot of attacks.
Again, just work on getting as many of these together as you can. This will be easier (and better) if you're a caster of some sort, but it's not required.
I've toyed around with decent volley archers in the past, and any of the following will be solid:
-A mounted Swift Hunter. Knowledge Devotion, skirmish, favored enemy, some limited spells. Ride on a mount so that you can make a full attack and move in the same round. Find someone to enchant your weapon for you, please - you won't have the stats to invest very heavily in UMD to do it yourself. It's not game breaking, but it's a solid character and shouldn't be underrated.
-Chameleon. Chameleon is very flexible, but works especially well for archers, because of their ability to cherry-pick spells that help out their cause. Chameleons can go the "traditional" archery route, or they can go with the Monk -> Soulknife -> Soulbow zen archer. The latter takes advantage of Flurry of Blows with the Soulbow for additional attempts, and focuses on Wisdom synergy, but ends up losing out somewhat due to less enchantment options on the bow. Also, they bloom as characters much later.
-Archivist/Cleric. I prefer Archivist, who can potentially pick up some very nice spells off of other spell lists (especially the ranger spell list), and who has natural synergy between Knowledge Devotion and Dark Tactics. However, you can't argue with a classic like the cleric archer, and Divine Metamagic alone makes the character worthwhile. But Divine Metamagic can also single-handedly break games, so use with care.
-Psychic Warrior. Honestly, I think psychic warriors make the best "out of the box" archers in the game. They've got the bonus feats to stand up to archery (which is a very feat-intensive fighting style), their powers are about as good as they come, and Zen Archery means they've got natural Wisdom synergy. If you want to get silly, you can pick up Metamorphosis via Expanded Knowledge (requires being an Outsider).
-Wizards, because they can do basically anything. They get access to the best offensive buffs in the game, including Polymorph (allowing for the dual-bow wielding Arrow Demon trick). I would go with a Swiftblade, personally. You get enough casting to make it worthwhile, but boost your BAB, and the free castings of Haste (without even costing you actions) combined with Swift Surge makes for some very nice synergy with a volley archer. The only downside is that you've got to take some useless feats to get there, and archers are always feat-hungry.
If you know for sure which direction you want to go in, I can give you more specific advice, but as you can see, there are a LOT of options here.