Hi Landmark,
Imagine a straight wing and its Mcrit is .70, when flying the relative wind is fully "sensed" by the wing as it is fully perpendicular with the leading edge. I will name it as WMcrit.
Now take the same wing and swept it back 30°. The WMcrit will not be changed but as the wing is swept, the wind will now be sensed as two vectors, one perpendicular to the leading edge and one parallel. In order to the perpendicular vector reach Mcrit the relative wind must be higher than it, so the aircraft can fly faster. How much faster?
Well, we can simplify things and assume that the max speed possible before WMcrit is reached is equal to WMcrit(original)/cos(sweep angle), in my example .70/cos30° -> .70/(sqrt(3)/2) = .0808.
In other words, our aircraft can fly up to M.808 until the wing "senses" M.70 and begins to have compressibility problems.
Off course this is an over simplification and other factors, including thickness, will influence Mcrit.
Cheers!