Īll versions of WMA released since version 9.0 – namely 9.1, 9.2, and 10 – have been backwards compatible with the original v9 decoder and are therefore not considered separate codecs.

These codecs were Windows Media Audio 9 Professional, Windows Media Audio 9 Lossless, and Windows Media Audio 9 Voice. In 2003, Microsoft released new audio codecs that were not compatible with the original WMA codec. Although earlier versions of Windows Media Player played WMA files, support for WMA file creation was not added until the seventh version. Prior to Windows XP, WMA files were primarily streamed using the Windows Media Source Filter ( DirectShow codec), later being removed in Windows Vista with the addition of Media Foundation. Microsoft first announced its plans to license WMA technology to third parties in 1999. Newer versions of WMA became available: Windows Media Audio 2 in 1999, Windows Media Audio 7 in 2000, Windows Media Audio 8 in 2001, and Windows Media Audio 9 in 2003. RealNetworks also challenged Microsoft's claims regarding WMA's superior audio quality compared to RealAudio. The former claim however was rejected by some audiophiles and both claims have been refuted through publicly-available codec listening tests. Microsoft claimed that WMA could produce files that were half the size of equivalent-quality MP3 files Microsoft also claimed that WMA delivered 'near CD-quality' audio at 64 kbit/s. It was later officially released as Windows Media Audio, as part of Windows Media Technologies 4.0. The first finalized codec was initially referred to as MSAudio 4.0. Malvar was a senior researcher and manager of the Signal Processing Group at Microsoft Research, whose team worked on the MSAudio project. The first WMA codec was based on earlier work by Henrique Malvar and his team which was transferred to the Windows Media team at Microsoft.

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