"Boop bop-a-doo-bop" also makes me think rather of Betty Boop than of Melkor
Apart from the humour, it is quite a cleverly done though: he wants to summarise Melkor's initial disturbing the music of the Ainur, which is a pure "LA LA LA!". Melkor counters this with all sorts of sounds that are very unlike "LA LA LA LA": "BUM BUM BA DUM!", then "Boop bop-a-doo-bop!" and finally "Bwam bardle ningle boom."
- I can sort of imagine how hard it would be to keep on "LA LA LA-ing" in close harmony with the rest when someone is falsely bardleninglebooming
next to you! Of course he eventually got a lot more heavy-metally than that ..