Try using
MSI Afterburner, its free and you can set this to record two separate audio tracks e.g. game audio and your voice. This works like FRAPS (one hit record, one hit stop) but as I've said, it records your voice separately from the game, so no need to worry about syncing. There's also an option where it balances out the volume of your voice and the game's audio.
Just one thing, MSI afterburner looks daunting to setup but it's not.