MediaElement playlist won't play all audio mp3's

Hi!

I’ve made a few playlists on my site with the WP core mediaelement player, using the audio shortcode. The mp3 files are self hosted. The problem is that often one or more mp3’s won’t play (in Firefox as well as in Safari and Chrome, although it seems FF doesn’t seem to have the problem as much as Safari).

If a file won’t play, the duration time shows ’Na:Na’. But often after a very, very long while it eventually comes up. The file size (between 1-5 MB) doesn’t seem to matter – which mp3 will play and which won’t varies per visit. Once all files play in one browser, they also play in another browser, which seems to point to a server or caching problem?

Additionale information:
– The files where uploaded using the Add From Server plugin
I’ve tried:
– completing the meta data of each file and simplifying the file names
– converted the files all to the same format (bitrate 128 CBR), originally they had different bitrates.
– even tried a caching plugin (Hyper Cache)

Site:
home page