1. Export Markers from Avid Pro Tools

Open the Markers panel in Pro Tools and export the markers as the .txt file. Alternatively, you can choose "File > Export > Session Info As Text" from the Pro Tools menu and select the Include Markers option to create the text file containing the markers with their location, name, and comments.

2. Use the "Subtitle Tool & Converter" to convert the TXT Markers marker file into a TTML subtitle file

Open the free Subtitle Tool & Converter, select "Avid Pro Tools > TXT Marker (.txt)" and set "convert" to "TTML". Generate and download the file. If you have problems converting the file, you can contact the free support.