You're missing my point: there's no lower limit on current that can cause fire. In fact, after I wrote the previous message, I realized all it would take was 20mA, not 100mA.
Diodes can malfunction too. A shorted diode (I've seen a few of those) act just like a piece of wire. Still sure you don't need a fuse?
I wasn't saying the wire would melt. It could get nicked and short + to -. Before it melts, it will start to glow. Giving off poisonous fumes. As it glows, it could ignite anything nearby. By the time that speaker wire melts and opens electrically, the room could be already on fire.
Take a look at the schematic below; what would happen if the 10 m long wires got nicked and shorted at the 'X'? (think like a door closing on it, or a dog gnawing on the cables) What would happen if you had a fuse? What would happen if you hadn't one? Do you see the total irrelevance of any diode? Do you see what would happen if the fuse was placed at/near the diode?
So, now you know too that you want the fuse as close to the battery as possible.
BTW, I'm 99.9% sure your setup wouldn't burn down your house. With a fuse, I'd be 99.99% sure. 25 years of experimenting and tinkering with electronics has made me very cautious.
Your dad is a clever man. Good thing you listen to him.