As a developer, it is impossible to search for music or sound effects for use in user created content. For example, if I want a boombox in my game, and I want people to be able to search for specifically music to play. Another example is if I want players to be able to hook specifically sound effects to things inside their house (such as a custom door opening sound effect).
These categories are split up in the “Create” tab, which using the network viewer I can see sources from toolbox-service in the form of https://apis.roblox.com/toolbox-service/v1/marketplace/301?limit=30&pageNumber=1&keyword=&includeOnlyVerifiedCreators=true, with “301” being sound effects and “300” being music. However, these asset types specifically require authorization in the form of a valid ROBLOSECURITY. This means that even if I were to set up a server (which I already have to do in order to get images, because GetFreeDecals might as well be unusable), I would need to take serious precaution as ROBLOSECURITY’s can expire or reset themselves if logged in from another area (as observed in people who try to use this in CI).
The safest solution I can do is use the search.roblox.com API (which I am already using for images, even though it appears to be magnitudes slower than toolbox-service), which only lets me filter by “sound”, not by “music” or “sound effect”, meaning a worse UX for dedicated “music” and “sound effect” searches.