OGG support...

[Makm]

Does WP 10 support .ogg format audio files?

Also, is there any app which supports it?

 

[N_LaRUE]

I think it's unlikely that W10 will play OGG automatically but I'm happy to be proven wrong. I don't think anyone can answer this question at the moment as it's still in TP.

As for apps.

Media Center 8.1 | Windows Phone Apps+Games Store (United States)

MoliPlayer Pro | Windows Phone Apps+Games Store (United States)

 

[RumoredNow]

I'd thought I had seen an article on that... Turns out it is FLAC that Win 10 will support natively.

Windows 10 to support FLAC audio format | Windows Central