MP3 is not supported in Godot. Until recently MP3 patents required licensing, which makes it very unfriendly for OSS projects. Some of the patents have expired, but it still remains a proprietary format, so it's unlikely to be supported in Godot any time soon.
Converting an existing file from MP3 to OGG is very easy using a number of different tools. Audacity is a popular open-source option.
Keep in mind, MP3 is a lossy compression format, so if the MP3 encoding is of low quality, the conversion will likely result in an even poorer-sounding file. You're best off starting with a high-quality lossless version of the audio.