Encoded the entire Premium OVA set of the first Fullmetal Alchemist series into high quality MKV h.264 format with both English and Japanese audio, and English Subtitles as well.

For playback all you need is the proper codec if you dont already have it. 
Combined Community Codec Pack (which I prefer) or the K-Lite Codec pack should give you all you need and more to play these back.

You should be able to hear audio just fine if you have one of the mentioned codec packs above. If you cannot hear any audio and do not want to install a codec pack for some reason, then you do not have and need the nero aac audio codec. As of today it can be found here. 
http://www.nero.com/eng/downloads-nerodigital-nero-aac-codec.php