I have recently changed the strings of my Yamaha c40 and all sound perfect, exept for the G string that sounds muffled , I can't find out why .
I would like to know if it is normal for a new string to sound strangely the first few days, or if i am making some mistake.

thanx and sry for my lousy english
In my experience, yes. Most strings need "breaking in" time in my opinion. I've had a set of flat tops on for quite a while now, and they're just beginning to round out in tone. I have to polish them frequently, though.
Sincerely, Chad.
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I think that the time that new strings sound their absolute best is the day after putting them on, and for about a week or two after that.