Here's a world much like Earth, except it's inhabited by a race of shapeshifters. On one side of the appearance divide there is an appearance much like a large Earth wolf; on the other is an appearance much like an Earth human.
The actual mechanics of the shapeshifting process are undefined, and I'm willing to do a little fudging there (but not really to the point of just saying "use magic"), but both forms should keep an exterior appearance actually resembling their respective species. So no humanoids with a large muzzle, nor brachycephalic wolves.
It's not too hard to find references to IPA charts, but I'm having trouble applying those to my particular question.
So basically, if we're allowed to make the internal anatomy as that of either a wolf, a human, something in between or something completely different, but are constrained by the external apperance of a human and a wolf and to the extent possible would like for them to also be able to make sounds resembling spoken English at least in human form (though with a strong accent is fine), then what sounds would be common to both forms of such a creature? For bonus points, rather than just making a list, also explain why those and not some other ones.
To clarify, these creatures (in either form) don't need to reliably fool someone with a training in medicine or anatomy (think doctor or veterinarian), especially upon close medical examination, into thinking that they are what they appear to be. However, they should maintain the respective outward appearance as much as is reasonably possible (small adjustments are allowed, but not major deviations, as illustrated e.g. by the human-with-a-muzzle example above).
Also to clarify, you are allowed to adjust the internal anatomy! So there is no need to be constrained by a wolf's (or a human's) voicebox, for example. What I want to avoid changing much is the exterior appearance. It is however best if the exterior appearance is as close to the only thing as possible that changes when they shapeshift.