Can you work in Thailand with a DTV visa if hired by a US company for corporate speaking engagements?

No, you can't. You'll need a work permit and the appropriate visa. The DTV doesn't allow you to work in Thailand for or with Thai entities. Even working as a speaker for a U.S. company in Thailand still involves dealing with third-party entities. It falls into a gray area and may possibly be tolerated if it strictly targets employees of that company (your own colleagues) for informational purposes (not a paid seminar) and doesn't happen too frequently. Best is to check with the local immigration...