My friend has one that works on a Bluetooth, works pretty good & I like it in particular since there's no plugs/adaptors/wires to deal with. It does use batteries though. Not sure the name or brand though sorry.
Homedics HX-GO33-9OS HMDX Portable Audio Speaker for most wall applications. Runs on batteries which is essential for multi-day routes. Cheap. Holds the ipod inside itself so there's less wires to catch on things. Having the volume and power button on the outside is essential too so you can quickly turn it down/off when the leader is calling down at you. Tip: when you're not using it, take one of the batteries out so there's no chance of the power button being pressed while it's being hauled from one belay to the next. Or just never turn it off and bring it with you while you clean.
perfect cheap wall speakers
Eco Extreme 3.5mm Aux Waterproof Portable Speaker Case for when you think there's going to be heavy rain or snow. The same as above but too heavy due to the waterproofing in comparison to the speaker above imo.
check out something called a minirig. Had a british friend bring one over and it effing rocks. will totally dominate other portable speakers. Its solid and not super fancy outside but its designed and made by a british physicist. It is way way loud and great quality. Only downfall...hard to find and buy. Only made and sold in GB and it runs about $180 US.