I realize this is old, but wanted to put an answer here for those that may come here in the future. According to Mitel's documentation, "you have to convert your music file into 8bit, Mono, non-proprietary G.711 format. The converted file must have extension .n64u."

To me, this means the "n64u" format is arbitrary and the system itself only cares about how the audio file itself is formatted. You should be able to convert your file using an audio program like Audacity or GoldWAV. Hope this helps.

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