Wild empathy is needed to communicate with animals who otherwise have no language. But if that limitation is overcome with speak with animals, does that open the door for a player to use diplomacy instead?
Wild empathy is not needed to communicate with animals, as Diplomacy is not language-dependent. A negative modifier or DC increase may apply to diplomacy checks made to influence a target who lacks a shared language, like all skill checks, at GM discretion (source). Wild empathy allows you to, instead of the normal modifiers and stats, use Cha+level to make the check. Adding your level to Charisma is only 3 worse than having max ranks in a skill, so unless you have specialized in or failed to take Diplomacy, Wild empathy checks will produce comparable numbers.
So, strictly speaking, speak with animals doesn't open the door to using Diplomacy on animals, but only because you could already try to do that anyways. It probably will give you a bonus or negate various penalties on that check, but the importance of doing so is entirely dependent on what modifiers your DM assesses in each case. The book seems to suggest a -2 modifier and possibly also a +2 DC as sort of a soft-cap to GM discretion negative modifiers, and the same in the opposite direction as the cap on positive modifiers, so speak with animals may give you something like +8 effective diplomacy on your Diplomacy check as an upper bounds and +2 as a lower bounds.
"I guess my confusion is this: >Use the table below to determine the effectiveness of Diplomacy checks (or Charisma checks) made to influence the attitude of a nonplayer character, or wild empathy checks made to influence the attitude of an animal or magical beast. p.72 PHB Does that mean that Diplomacy is only effective on people, not animals or magical beasts, no matter what?"
No. Let's look at that text a little more carefully. We use that table for two situations:
- Diplomacy checks made to influence the attitude of a nonplayer character
- Wild empathy checks made to influence the attitude of an animal or magical beast
So, there are some checks made with Diplomacy or Wild Empathy that the table cannot be used for, or at least is not indicated as used for:
- Diplomacy checks made to influence the attitude of a player character (since any character that is not a nonplayer character is a player character by definition)
- Wild empathy checks to influence the attitude of anything other than animals and magical beasts
- Diplomacy and Wild empathy checks made for other, non-attitude-changing reasons.
And that's it. The crux of the issue here is that animals are NPCs (well, usually. If one of the players is playing it then that's different); they are just a special kind of NPC that's also an animal. Just because you can only use Wild empathy on animals doesn't mean you can't use diplomacy on them. It's just like how feats that only let you try to do things you were already able to do don't actually mean you couldn't do those things in the first place.