relative URL link to local file in PDF document

I sell an ebook that contains relative reference URL links to mp3 files that come bundled with the download. These mp3 files end up in the same folder as the PDF ebook once the user unzips everything, so the links are dead simple (<a href="filename.mp3">click to play audio</a>). This works fine on Mac or PC. I can't seem to figure out how to make it work on an iPad/iPhone though. I can get external links to work, via the PDF Expert app, but everything I've tried fails to allow the end user to click links in a PDF that point to a locally saved file.

Anyone know a way to accomplish this? I've tried obvious things like "./filename.mp3" and "file:///filename.mp3" to no avail.