problems converting audio to video (for use with a seekbar)

Hi all, my end goal is to insert a piece of audio in my Storyline 2 content and give the user a seek bar so they can pause, stop, rewind and play the audio. I've read various threads here already, and have already decided not to use the built-in seek bar from the template as it ruins the look of the module. The only other option I can find is to convert the audio to video, then insert it as a video (which then gives me a video seek bar). However, no matter which converter I use (online-convert.com, onlinevideoconverter.com, zamzar.com etc) or which video output I choose (h.264,h.265, mpeg1 or 2 video, or any other mp4 format) when I try to inset the video I get the message "video.mp4 does not appear to be a supported video format." 

I've used mp4 videos in other Storyline courses before with no problems, so I'm assuming there's something happening during the conversion process that's blocking its use. I did read a thread on codecs, but couldn't understand it at all (as I'm not a techie). Can anyone suggest what else I should be taking into account when converting a file? Thanks.