Acelp.net Audio Codec is a speech codec for Windows Media Player, developed by VoiceAge - the World's Premier Supplier of Speech and Audio Codecs.
Microsoft clearly recommends using the ACELP codec by VoiceAge. This is the same codec RealNetworks uses to encode speech. "Indeed, when encoded at just 16 Kbps (one-fourth the size), the ACELP voice codec nearly matches MP3 quality at 64 Kbps!"