As a developer, it’s impossible to upload audio files for free.
I’ve personally spent over 100,000 Robux on audio upload fees. To put that cost into perspective, it would take at least 4 years of accruing monthly stipends from the highest level of Premium to earn that amount.
Developers who choose not to upload their own audio are limited to sounds that have been uploaded and shared by other users. For me it is very difficult to find suitable music or sound effects in the Library due to a lack of search tools (such as categories, or filters) and if I do find a suitable sound, the uncertainty over who owns the rights to the sound usually prevents me from using it in final production.
This leads many developers to source their own sounds, but in order to pay the upload fee, Robux must be bought directly or earned through sales, which means it could be a deal-breaker in the case of a new developer creating their first experience.
These kinds of paywalls can be significant barriers, and they should be removed wherever possible in order to maintain the awesome, sandboxy, free-for-all nature of Roblox’s engine.
EDIT: I found this feature request very relevant to this thread, and it posits an interesting compromise that would allow developers to expediently create game atmospheres by using default Roblox sound banks. Not a complete solution to the problem of audio upload costs, but very helpful!