There is only on city in Thailand. That is Bangkok.
In Thai language, city is "Mahanakorn". There is nowhere with that suffix in Buriram, Surin or Sissaket.
I have lived in Buriram for over 24 years. I also regularly visit Surin. I speak Thai and I understand Khmer language to a point and can assure you in the town centres I mostly hear Thai. As one moves away from the town, heading north the dialect changes mainly to Isaan Laos. Heading south there is a mix of Isaan Laos and Khmer. In some villages around Satuk and Chom Phra area you'll hear Gui, as that this where the Elephant herders are from. There are a few other areas where there are Gui speakers, Srikoraphum being one, but not so many.
The reason Thai is more prevalent in the town centres is those that speak Isaan Laos cannot understand Khmer. The same cannot be said the other way round. Hence, if there is a group of people who speak various dialects, Thai language is the means of communication. This does not happen in the villages as they are generally single dialect, plus Thai language, places.
I don't recall saying dialects are not spoken in the town centres. I said they are "mostly" spoken in the areas outside town.