Nobody can tell you anything without proper system info or other technical details like what version of AE, exact render settings and so on.
In the future please provide us with some more info on your computer specs, AE version, exact export setting - it will be really helpfull.
But issue with audio is already commonly known issue so:
If you did everything as it should be to export audio that can mean that you use AE version 13.5.1. If you use that version and try to render to avi and also to mfx aiff with Add to render queue... - you will not have audio.
That is the issue that is known and will be resolved as soon as it is posible.As a work around to export video with audio or only audio you should use Adobe Media Encoder to render Avi, mfx or wav, mp3. You can do that by chosing Composition > Add To Adobe Media Encoder Queue. You can also render your composition in earlier version of AE.
Thank you for your answer. I use the latest After Effects CC. I guess it's the issue you said... I will try to render it in Media Encoder. I hope I will not loose quality! Thank you for your respond.
If you will render with the same codecs and the same codecs specs as you did in AE - everything should look the same.
Thank you all!