The "Salve Regina" is a traditional Catholic hymn, but there is no specific piece attributed to a composer with that title and the term "simplex."
It's possible that you might be thinking of a specific version or setting of the Salve Regina, or that the name you are referring to is slightly different.
Could you please provide more information about the piece you are looking for? This could include:
* The composer (if known)
* Any other parts of the title
* The context in which you encountered it (e.g., a specific church, concert, recording)
With more details, I might be able to help you locate the piece you are looking for.