FLAC, both, embedded in the Ogg container or in its native container format is supported for applications where lossless compression is required, for example projects where a file has to be edited multiple times.
To convert your audio to one of the allowed formats, you can use most video converters, they can also handle audio without video or extract audio from video. The command line tool ffmpeg is available for various operating systems and internally used by many tools with a graphical user interface. On Linux check out OggConvert.
Creating and editing audio
When recording sounds or spoken samples with your own equipment please take care of the following aspects. Make sure to eliminate all background noises. Though, it is possible to remove unwanted noise from the audio stream afterwards, it is better to minimize your editing work. Furthermore it is advised to use a high quality microphone. Built in hardware microphones, especially those from laptops, are considered to produce low quality recordings which include clipping, noise and therefore a low SNR.
Audacity is not only able to record audio, but also offers a multi platform editing tool for an unlimited number of audio streams.
A note about copyright
Uploaders must ensure they have gathered all rights necessary, including but not limited to recording rights, rights by the interpret(s), by the composer and of the lyricist, whenever applicable. These individual rights should be specified separately on each file description page of audio and video files.