Is silence regarding the manipulation of expense receipts by some employees considered participation in sin, especially given the company's lack of interest in those expenses?

As long as you perform your work as required, there is nothing held against you, and there is no sin upon you if some employees engage in fraud and forgery in invoices without your knowledge. You are not obligated to inform the company based merely on suspicion. However, if you come to know of a theft occurring through fraud and forgery, you must prevent it if you can, or inform the officials if you do not have the authority to do so.