[RPG] Does alter self also change the caster’s voice

Stat Changes

It seems very clear that you apply no stat changes.

This ability functions as a polymorph spell, the type of which is listed in the creature's description, but the creature does not adjust its ability scores (although it gains any other abilities of the creature it mimics).

There are no qualifiers here -- it says the creature does not adjust its ability scores, so it doesn't! The part of polymorph that you're looking at says that you

adjust its ability scores to one of these two sizes using the following table

This is exactly what the change self ability tells you to ignore.

Even though it doesn't use the word "adjustment" in the actual text of alter self, the word would generally mean any change to your stats. (And it would be a bit weird if shifting into a smaller form increased the strength of a Titan.)

Other abilities

There's nothing here to modify how polymorph spells work:

While under the effects of a polymorph spell, you lose all extraordinary and supernatural abilities that depend on your original form

Note the qualifier. You only lose the ability if it was contingent on your original form. In many places the DM will have to make a judgement call on whether that applies. But obviously, say, a manticore that shifts into human form can no longer use its Spikes special ability. The rule is intended to rule out nonsensical situations, so the change shape ability doesn't alter these rules at all.

The Disguise bonus is only relevant if you're making a disguise (and therefore a Disguise check) and using (e.g.) Wild Shape to help with it.

Otherwise, the Disguise bonus is irrelevant and no, does not imply that people can just see through the Wild Shape using Perception against the normal Disguise rules.

Notice that nowhere in the rules for Transmutation (polymorph) does it say you have to roll a Disguise check to have or maintain any of the effects of being shapechanged. The bonus is only mentioned so it can be used in other circumstances that do require a Disguise check.

Thinking about a cat hasn't been helpful as an example, so let's start fresh with a different example and consider a bear.

Case 1: You're Wild Shaping into a bear because you want to bear around a forest. Here, Disguise stuff is irrelevant.

Case 2: You're Wild Shaping into a bear because you want to fool the local goblin tribe that you're their totem bear, Great Paw, and you want to give them an "omen" that will make them move out of the forest. Disguise checks will be necessary here to make you look like Great Paw, and Wild Shape gives you a +10.