Soul Voice doubles the value (For songs that Soul Voice doubles potency for) after everything else is applied.
E.g For Minuet V
Base Cap is 62
G.horn and Empyrean Body +2 is +17
5/5 Minuet Merits is +5
10/10 Minuet Job points is +10
Regular Minuet V = +94 Attack
Maracto'd Minuet V(1.5x) = 141 Attack
Soul Voice Minuet V(2x) = 188 Attack
(This isn't counting the empyrean set bonus which would give an extra 2 Attack that isn't affected by Soul Voice since it's just a straight +5 STR)
BG wiki has most of the values listed for songs and modifiers under their category page.
http://www.bg-wiki.com/bg/Minuet for Minuet.
As for songs Soul Voice doubles the potency of most songs but not all. Going from memory, someone can correct me if I make any mistakes.
Debuff
Requiem - Doubles potency
Lullaby - I'm not sure anyone has ever quantified this I think it improves magic accuracy in some way.(?)
Elegy - Doubles potency. Although caps at 50%.
Threnody - Doubles potency.
Finale - Same as Lullaby(?)
Virelai - Same as Lullaby.(?)
Nocturne - Potency or Duration? I'm unsure on this one.(?)
Buffs
Paeon - Doubles potency
Ballad - Doubles potency
Minne - Doubles potency
Minuet - Doubles potency
Madrigal - Doubles potency
Prelude - Doubles potency
Mambo - Doubles potency
Debuff Resist Songs - Doubles potency(?)
March - Doubles potency (Players still subject to max attack speed caps of 43.75% for magic haste and 80% combined delay reduction)
Etudes - Doubles potency
Carol I - Doubles potency of elemental resist.
Carol II - Increases the nullification rate(?) Unsure of this.
Hymnus - Doubles Duration
Mazurka - Doubles duration
Scherzo - Doubles duration
Sirvente - Doubles potency(?)
Dirge - Doubles potency. Doesnt bypass -50 enmity cap.
One other thing to note is the duration increase from Clarion call job points is added after Troubadour so that doesnt get doubled. Or atleast the last time Creelo checked that was true.