'08 Fusion with intermittent deep horn-sound when radio or CD player is on. Sounds like an SOS in a tunnel, or a horn on an 18 wheeler. Dealer is mystified.
From your too-brief description we might surmise dealer isn’t really trying. Maybe or maybe not. Has he swapped out your stereo unit with another one? That is step #1.
If the dealer can’t fix it, you get a new car. That’s what the lemon law is for. Find out what the specific requirements are in your state and keep good records. You need to have a receipt for each visit. Did you get one for the last visit? How many times have you been to the dealer for this issue?
Web search for “lemon law” and see what the requirements are. I consider getting a new car as a last resort, but you need to be ready. In MD about 15 years ago, you needed 6 verified visits and a letter to Ford before the last one notifying them of the issue.
Keep after the dealer. This should be a warranty fix, and it sounds like he’s stalling and/or clueless. Make sure you have a copy of the work order from when you first reported the problem so they can’t weasel out of warranty coverage.
Radio and sound systems are really not a major priority for dealer mechanics and they may not have a lot of experience with this sort of thing, but the zone rep will have an expert on call, so get this bumped up the line to bring the real expertise to bear.