If you are using a standalone application to convert WAVs or AIFs to MP3 or another compressed format, try to find an application that shows you the settings and better still, lets you specify a bit rate. Free apps (sometimes also paid ones) can just give you a big “convert” button, but you don’t know how much data is going to be thrown away during conversion. Having options like “small, normal or high quality” is better, but a manual control is ideal. Use 320kbps for maximum MP3 or M4A quality. Stick with stereo, constant bit rate and original sample rate for best results when converting.
Social media sites and many other websites that allow you to upload music files generally perform their own compression to get the audio into their streaming format, and this is beyond your control. The best you can do is feed them the highest quality originals they allow, and hope that their algorithms do their job properly. Alternatively, sites like Bandcamp accept uncompressed files and then make those and compressed versions available for sale, with a full breakdown of the audio quality settings that they use.
If you’re working with a lot of files it makes sense to choose an application that can batch convert them using the same settings. DAWs usually let you batch export tracks, though it’s rarer to find the ability to batch process files. Audio editors like WaveLab and Audition do have this feature though. Pro software like Apple’s Compressor is more focused on video, but does have a wide range of audio compression options, which are fully tweakable.
Compressing at really low bit rates (audio books have been known to use 64kbps) is a false economy since although the files will be smaller, they will sound terrible. The top end becomes muddy and high frequencies start to sound like they have had a phaser applied to them. Really these days there’s no reason to use anything lower than 256kbps unless you have a very good reason to do otherwise.
There are still many kinds of audio compression formats available, though it makes sense to focus your energy on using the most widely popular ones if your music is going up for sale or distribution. MP3 fits the bill, and it’s probably worth avoiding platform-specific formats, or at least if you’re going down that route, cater for all platforms. Everything can play MP3s, Apple’s M4A is problematic on Windows since it uses the AAC codec. WMA files can only be used on a Mac in third party software like VLC. FLAC files aren’t natively supported on the Mac but are on Windows, though there are plenty of third party players that can read or convert them.
Learn more about digital audio compression here.