I was given an old acoustic guitar, A Eterna EF-15. I put a new set of strings on it and tuned it up, but it sounds horrible. I've checked it over and everything looks fine, it just sounds awful. It was stored in a basement from what i understand, the basement is heated.

So what could be wrong with this thing?
lol, well yes of course that. But i mean it don't sound right, i've heard Eterna's before and granted they didn't sound the best, but they sounded a whole lot better then this.
wrong. as the wood gets older, it has more time to cure and dry out, therefore giving it a richer sound (pretty sure on this). something else might be wrong with it.
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funkdaddyfresh is a real American
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but it probably didnt dry out, it wa in a basement, generally humid places

He said his basement was heated.. so I'm thinking the wood is dried up like crazy.

FZRaven, consider putting a humidifier in your guitar bag. Or even leaving your guitar near a room humidifier.. just to get some moisture back into the wood.