Zachiel's answer is correct, but here's a walkthrough just in case.
Consider not streamlining encounters...
It sounds like the group knows the rules but dispenses with some to speed up play. This is totally understandable. But it also sounds like the players might need more granularity to realize their play style.
In our last session the party was ambushed by four drow...
The drow made Stealth skill checks when the party was near enough to make Perception skill checks and the drow all won. It's the surprise round. Everyone makes initiative checks. The PCs are unaware combatants, so they won't be acting during the surprise round.
The PCs, at this point, are unaware combat has begun.
One drow is willing to talk but three of the drow ready lightning bolt spells...
At this point any drow can choose to reveal their positions to parley or, if in an appropriate position, parley from their hiding places (likely also revealing their positions, or at least giving the PCs another opportunity to make Perception skill checks). Breaking stealth to reveal themselves to the party probably takes the drow's actions during the surprise round. Taking a free action to speak is possible for both sides, so a parley during the surprise round is totally possible assuming negotiations are short ("Speaking more than a few sentences is generally beyond the limit of a free action").
Any of the drow who don't take other actions can take a standard action during the surprise round to take the ready action, setting the action as cast the spell lightning bolt and the trigger as When a PC takes an aggressive action.
- The spell lightning bolt may be an unwise choice unless tactical circumstances permit the drow to avoid catching any of their fellows in the spell's rather large area of effect (A targeted spell might've been a superior tactical choice.) But they're drow; maybe they want to catch their fellows accidentally in the bolt's area.
- This is a really broad trigger for the ready action, and the GM may mandate that the drow need to be more specific as to the nature of the trigger, maybe requiring instead the trigger be a specific action, like If a PC casts a spell or If a PC draws a weapon.
The sorcerer started readying a fireball. The drow view this as aggressive so they cast multiple lightning bolts...
The GM's made it clear to the players beforehand that the process of readying an action is obvious and that the outcome of that ready action is equally obvious. That's not the case by default, but, since there is no case by default, these house rules are as good as any. Note that a successful Perception check versus the drow who had also taken the ready action would've revealed that they had taken the ready action to cast lightning bolt.
- This GM is not entirely comfortable with a house rule like Foes who can perceive the creature know when a creature has taken the ready action and know what action the creature will take with that readied action. Preparing spell components to cast a spell is a free action, so it's not necessary for a creature to reveal to his foes that he's taken the ready action to cast a spell; the components can be prepared after the trigger action's come about.
- The obviousness of the creature having readied an action can also be disputed. A creature that's readied an action doesn't take a penalty to its AC, for example, nor to any checks, and hasn't taken any other actions, either. That the opposition nonetheless (perhaps intuitively?) knows the creature's taken the ready action strikes this GM as weird.
...And, instead, taking encounters step by step
- The drow find hiding places. The PCs approach a position from which the drow may be perceived. PCs make Perception skill checks opposed by the drow's Hide skill checks.
- Everyone makes initiative checks, but only aware combatants (the drow and any PCs who win the Perception versus Hide opposed checks) can take actions.
- In initiative order, one drow emerges from his hiding place to parley with the PCs. The remaining drow ready to cast lightning bolt if a PC takes aggressive action.
- Aware PCs also act (or, possibly, delay).
- PCs that take aggressive action are subject to the drows' readied actions.
- PCs and drow take their actions normally.
- If the PCs take aggressive action before the drow's actions come up this turn, the drow's ready actions trigger and their initiative counts are reset. If the PCs don't take
aggressive action before the drow's actions come up this turn, the drow can ready again (either the same way or differently) or take actions normally.