Board Thread:ROBLOX help and discussion/@comment-30623141-20161228052836/@comment-28882506-20161228053451

Uploading can easily be done through the roblox.com website. To do this: Note: Audio uploads must be less than 6 minutes, smaller than 10.0 MB, clear and easy to understand. This means that audio cannot be sped up, slowed down, or had its volume excessively altered.
 * 1) Log into your account on roblox.com
 * 2) Click Develop in the blue bar at the top of the screen
 * 3) Click on  Audio
 * 4) Click Browse, and select an audio file
 * 5) Once selected, click  Estimate Price  to determine the Robux cost of your upload
 * 6) After you make your selection click  Purchase for XR$   to finalize your purchase

-ROBLOX Official Wiki

Hope this helped! ~Evy