base64-audio-mime/index.js

// index.js
const atob = require('atob');
const Buffer = require('buffer/').Buffer;

const mimeTypes = {
    mp3: 'audio/mpeg',
    wav: 'audio/wav',
    ogg: 'audio/ogg',
    aac: 'audio/aac'
}

function getAudioMime(base64Encoded) {
    if (base64Encoded.startsWith('data:')) {
        const found = base64Encoded.match(/data:\S*;base64/g);
        return found && found[0].slice('data:'.length, ';base64'.length * -1);
    } else {
        const prefix = atob(base64Encoded.slice(0, 60));
        const found = prefix.match(/(mp3)|(wav)|(ogg)|(aac)/gi);
        if (!found) {
            return null;
        } else {
            const type = found[0].toLocaleLowerCase();
            return mimeTypes[type];
        }
    }
}

module.exports = {
    getAudioMime: getAudioMime
};