Lerp is overly complicated and I find it quite stupid that C2 doesn't have a basic fade music effect.
try this :
1. use a global name it FADE
2. when (your_conditions) -> every tick, set volume to FADE
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Volume fade in-fade out..
rex_audio_helper
http://c2rexplugins.weebly.com/
Same topic here: viewtopic.php?f=146&t=117246