I've attached a simple file to show some of the Visio text quirks. When dealing with Visio text, generally, it's all about "characters".
If you open the file, there's a single shape with regular and bold text. Open the shapesheet, scroll down to the Characters section. You will see multiple line entries. The left column is the character count. Believe it starts with 0, not 1. Could be wrong. Two of the rows have style 17. These are your bold characters.
Now open the VBA edit window <alt> + F11. Under Modules>NewMacros you'll find 2 macros. Each of those selects a line of bold text. You'll see a correlation between the code and the shapesheet.
So, you need to search the characters section, find a row that has style 17, then copy the set of characters that span that row.
HTH
Wapperdude
BTW, code was generated by the macro recorder. It really is a handy tool.