I first noticed this odd pronunciation in the second opening of code geass. The singer repeatedly pronounces the word かんかく as something that sounds more like かんかう, missing the 'k' sound.
I noticed it again more recently, in the opening of mondaiji, in which かすか is pronounced as かすあ.
I was aware that vowels could be skipped in pronunciation, but these are the only instances I've seen this happen with consonants. While there don't appear to be any other words you could easily mistake these two for when pronounced this way, I'd have thought it'd still cause at least some confusion to pronounce them like this, even if it may better fit the music.
How common is it to remove consonants like this? Does it happen with consonants other than 'k', and are there any words in which it's common to leave a consonant out (like how it's common to leave out the 'u' in 好き)?