In an uncial episode of The Language Files (previously), host Tom Scott takes an interactive look at the International Phonetic Alphabet Chart. Scott explains how the chart categorizes the naturally occurring letter sounds within the Latin-based Alphabet, such as the English Alphabet. He also explains how the chart also leaves a space open for the rare possible new sounds that are difficult, but humanly possible and grays out the boxes for those sound that can’t be made at all.
Some of those empty spaces have white backgrounds, and some of them are completely grayed out. The ones with white backgrounds are possible for humans to make, but they’ve not been given a symbol, because linguists have never found them actually being used as sounds in any of the world’s languages. Sometimes they are found to exist by some researcher somewhere, and if that happens then the International Phonetics Association will add a symbol. The last time that happened was in 2005. But then there are shaded areas, the shadow lands, the sounds that are theorized to be impossible to produce.