I have the issue with all headphone audio output: Music, Google Play, Radio, Videos and Youtube. The issue is present with three different headphones: Sennheiser Momentum, Sennheiser PX360 and the LG earplugs.
Tried a LG G2 in the local store today (4.2.2) and the headphone audio output was perfect, just as it is supposed to be. I have sent an email to the local LG support-site (Denmark), hopefully they have a solution.
Is it possible to downgrade to 4.2.2 ?. It is also annoying that it is longer possible to disable Qslide in the notification area on Kitkat.