when trying to upload mp3 files to neocities you will inevitably run into the problem of a paywall. non paid accounts can only upload files of specified types. the reason given on their website is to prevent them from becoming a file dump host, which seems reasonable. the solution seems obvious, to use an actual file dump host instead of neocities. they even reccomend doing this, giving the examples of youtube and soundcloud. however the issue with souncloud for audio hosting is that they do not give you direct access to the audio file, so you cant use it in your HTML code for background audio. This guide will give a solution using mediafire instead.
Make a mediafire account on their website and upload the files there. im pretty sure most of you can manage this yourselves.
right click on the file and select download as shown in the image.
right click on the download button and press copy link location shown in the image. this may vary based on browser.
insert the link into your code. shown below is an example from my website.
this method will probably also work for other filetypes that you want to host. i take no responsibility for what you upload.