It isn't explained in that old thread, it goes like this.
The original stand-alone AoE, and stand-alone RoR, on one CD each, have in addition to game data files, music in CD-audio format, which takes over half of the space on the respective CD-s.
The CD is not Red Book format, it's Orange Book (a format which combines both Red Book which is when CD contains nothing but CD Audio, like the CD releases in the music industry, and Yellow Book which is when the CD contains nothing but data files).
The Audio portion of the CD is in CD audio format however and because of this you will not find this music as files on the CD with any regular Windows utilities. The music data can outside the game be usually recognised by programs that are used to handle CD-audio format (your average music CD). In Win98 there was a nice little program CD-Player which could do it. In XP and Vista I'm not so sure.
The music is also available in MIDI format, which is a complete recomposition of the music in entirely different format that sounds completely different. This format can be found as files on your computer after the install. This format I believe is used in multiplayer for the players that do not have CD-s but can play multiplayer when their mates do have the CD. It is also used in the demo versions of the games.
Unlike the standalone versions, AoE Gold crams both AoE and RoR data files onto one CD, and so there is no room left on this one CD for even one CD soundtrack, let alone two of them. Because of this, it always uses those extremely small MIDI files for music instead.
IN ADDITION to this, different sound cards and different setting may lead to the MIDI files themselves being played very differently, which only adds to the confusion. In Win 95 and 98, if you select Sound in Control Panel, then under the MIDI section you can usually select between two MIDI devices, one of which makes AoE music sound very bad because it wasn't created for this device (likewise, music which WAS created for this device will sound very bad with the other device). Between different sound cards there might be yet further differences even when using the same device, although these are much smaller.