Migrate SQL Server backup to MySQL for a Wordpress site
Good luck.  You'll need to do way more work than just a "restore".  

Essentially, you'll need to create a custom pipe to convert the data in the SQL Server database into the table structure used by WordPress.

Even if the table structure was identical, and it isn't, you can't "restore" a .bak file into a MySQL database.  The .bak does not contain SQL statements; it is a binary representation, page by page, of the SQL Server database.

Presuming there aren't a huge number of articles, it is generally considered easier to manually transfer them into WordPress using the interface.  Welcome to the thrill of vendor lock-in.

If you have access to a SQL Server, you might try restoring the .bak file then exporting the desired data as SQL Statements using the "script database" facility.  You might then be able to do a transformation on the SQL Statements to make them "fit" the design of the WordPress database.  I'd be very skeptical that this would be easy, however at a certain point this will take less time than manually recreating the stories in WordPress.