And hey, if you create better sounds for it, why not contribute it back to that open source project?
You could ask for your name in the credits, and give them non-exclusive permission to use those sounds and music (depending on the project's licensing requirements), while also not giving everyone permission (just that project). The game itself could be a subtle billboard ad which you might get one or two future contracts or contacts through.
Or, make a sound mod for a popular game like Minecraft, which encourages mods. Make sure to name the mod something that leads people to google about you like, if you were nsmadsen, "Minecraft Madsen Mod" or "Minecraft Madsen Music Mod", so people would be curious what the "Madsen" part of the mod refers to, and also so people wanting to download this uber-popular mod would google the mod, and find the mod hosted on your studio's website and next time they need music or sound, they'll have your studio name stuck in their head.
Bethesda is also supportive of mods for their Elder Scrolls series. A mod that is high quality and popular in their community, like the (fictional) "Madsen Skyrim Music" mod might reach the attention of developers of the Skyrim game itself, and next time they are rushed for deadlines and need to outsource some musical work they might give Nathan a call.
I think this would be a better idea than modding a commercial game from a studio or publisher that has twitchy-fingered lawyers, plus mod-friendly games usually have better modding documentation and modding communities that would be very mentally supportive and practically helpful in what you are doing.