In terms of civil liabilty, or who owes the money, unless you qualified the endorsement of the payroll check in some fashion, e.g., by writing "without recourse" above your endorsement on the back of the check, Wal-Mart can hold you responsible. In turn, however, your employer is liable to you for the money. Even if you did write "without recourse" on the check, you may still have liability to Wal-Mart under certain other warranties you may have been deemed to have made.
What is Wal-Mart doing?
If you mean criminal liability, unless you knew the check was bad, I wouldn't worry to much about it.
You ought to consider contacting the Missouri Department of Labor and reporting your employer if this is a continuing problem.