I'm not sure it's the case with things like this, and I haven't encountered it so far ...
But, in some songs or games or manga or whatever, sometimes they use different kanji, or 'special' kanji, or normal kanji with a different-than-normal reading for ... Well, I suppose artistic reasons. (In D.Gray-man, they use '神' and put in furigana イノセンス (ehm if that's spelled correctly~ 'innocence' next to it. While they normally just put イノセンス, I've seen other kanji used for 'イノセンス' as well.))
And I think it's incredibly funny when they put the normal kanji for something, and give an English pronunciation for it in furi. I guess that's to give the meaning for the readers but also have the cool English word?
As for 思 and 想... maybe it's just preference... or because one is more 'special' that they are used at times. (Like English writers sometimes like to use rarer words ... just because?) ...
I have to say though, I haven't come across these examples (聞く and 聴く, 思 and 想) so I'm just guessing. ^_^;