I tried using Dropbox links in my cache manifest, but it didn’t work.
I tried NOT using cache manifest and simply:
<audio src="dropbox url" autoplay="true"></audio>
It doesn’t play the sound either. If I point src to a file within the project or on the hard disk, it works. Are there any free services for hosting such things?
I don’t know of any free and legal services for hosting audio content. I’d recommend looking into cloud-based storage options (AWS S3, Rackspace Cloud Files, et cetera). They typically have a cost based on the amount of files you have stored and the amount of data transfer per month.
You should be able to use external URLs in your cache manifest. However, it may be possible the URL you’re trying to use has response headers that prevent it from being used in such a way.