There were some moments in my playthrough where I just loved the music, and even more going through the soundtrack afterwards. I've even just been trying to replicate the melody from The Power and Last Light on the keyboard... although I'm a wind player, so these 'chord' things are a bit tricky to get my head around. A few of my favourites:
The Power - I almost consider this the theme to the game, given how many times the melody comes up in different places. Love the piano intro in the credits version, and the tune
Alfira's song -
Firstly, because it's the first song you hear in the game. But it's also special to me because it helped me understand my bard's personality. Yes, I will sit down and work through your song with you. It would have been brilliant if she writes a song about you by the end of the game!
Last Light -
I listened after finishing the game, and I love how lonely yet hopeful that clarinet solo sounds. It always makes me think of a single lamp flickering in the dark, which is perfect for the Last Light. And I always hear the tired quartermaster saying "We don't know when 'next blow will come"
End of act 2 -
When I camped for the last time before reaching Rivington, and the same camp music as before started up, but then someone started singing... that one gave me chills. I can't remember if it was the Song of Balduran, which admittedly I'd heard enough of after several nights at the Elfsong, but getting it that first time was just amazing.
I'd also like to say how much I like the fact that you can join in with every bard in the game. It would have been so easy not to implement that, but basically every time I found a performing bard I'd choose the instrument most unlike theirs (I had violin, flute & lute) and join in; and it always sounded excellent (although the audiences get a bit annoying with the sudden whooping and shouting). Does the game actually have melody and accompaniment parts for all instruments? I mean, if I start playing on my own I must be playing the melody, but I remember the bard in the guildhall plays the flute, so I joined in on the lute and I think I was playing an accompaniment part.
