[RPG] Could a phylactery of a lich be a mirror or does it have to be a box

Create undead does not specifically state that the soul/undead/etc is of the original creature; it just states that a dead body is required.

This leaves it a little open to interpretation as to where that hapless soul is going to come from to fill that corpse that's being filled.

However, more powerful undead, eg Mohrg are the "animated bodies of mass murderers", so although you can create a Mohrg at a suitable high enough level the component is still "a body". So this would imply (to me) that when an undead is created a suitably evil soul is dragged into the corpse. Otherwise if they're trying to animate the corpse of a pub landlord or something into a Mohrg then there is a disparity between monster description and the soul of the person being "used" as a component.

My personal take would be that either the soul of the prior unfortunate person who was living is dragged screaming back into the body when the undead is created (if they are "evil" enough and not protected somehow) otherwise a malign spirit is instead called into the corpse of the undead; once killed the spirit is destroyed and banished from the plane for the atypical 101 days/moon phase/etc and re-raising it (if allowed) would summon another spirit instead with it's own memories and whatnot.

As a footnote, the advantage of speak with dead is that it's a non-evil spell; so granted summon undead could get you the same result, but you'd have to be cackling and stroking your white cat and so on if you wanted to do it.

A destroyed creature with the template class lich refoms up to a few miles from its phylactery

The Wizards of the Coast Savage Progressions Web column "Lich and Weretiger Template Classes" includes in its description of the lich template class's class feature phylactery the following text:

Once the phylactery has been completed, the lich can avoid permanent destruction as long as her phylactery survives. If she dies or is destroyed, she reappears 1d10 days after her old body's death. She gains her new physical form by grafting her undead spirit to a humanoid corpse, mindless undead, or some weak-minded creature within a few miles of her phylactery. The new body has all the abilities and powers of her old one, though any items she used to carry are lost (probably taken by those who slew her old body). Likewise, any spells or effects bound to her old body with permanency do not spontaneously appear on her new one. Most liches who recover from death spend a year or more tracking down their items and learning more about their attackers, and it is not unusual for a lich to wait decades before exacting her revenge.

Emphasis mine. Whether the same is true of all basic liches is unknown, but without additional guidance this seems a good enough rule as any.