Hi. As long as you upload it somewhere and state it is 100% public domain, the copyright rules for most sites then fall into the copyright rules of your country.
The only two I know for sure are U.S. and Colombia. In the U.S. copyright starts the moment of publication (posting online with a date = publication) and is 70 yrs past the date of your death. In Colombia it is 80 years, and you can transfer the rights to a Public Doman entity simply by gifting it in an email (or something else that verifys the time/date).
More info about U.S. copyright here....
https://www.copyright.gov/circs/circ15a.pdf
For legal reasons, you'll want to include a license with your upload. Stating something like this will protect your rights!
CC0 Public Domain by <YOUR NAME/Nick/Handle>
Free commercial use No credit needed <description of content -- a song about my dog, music, guitar, singing, piano>
No Copyright, Royalty Free
License: Creative Commons Attribution license (reuse allowed)
Cheers,
Cexmental