Use the MYVOCABU chrome extension, and in OPTIONS set up the hotkey “Translate selected text when I press button:” (I like using the SHIFT key for this)

1. highlight the Thai text/sentence you want to hear (optional: transliterate page with Mekongeasy first)
2. press SHIFT (or hotkey you set up in Options) to get English translation
3. click on the  symbol to play the selected text in Thai