Item -
Listening Horn
Properties -
Material: Brass
Amplify (5) 1 charge per use.
Charges: 40
Sugesstion-
Add item restriction "Only usable by: Bard"
Reasons-
As it stands, this item not only gives 40 charges of a +20 listen spell, but it actually lasts longer than the scroll, being a caster level of 5. It's extremely cheap and drops quite often, and has become an easy method to boost a skill with little drawback. This is a spell unique to the Bard class and provides an amazing bonus to listen skill. I think 40 charges are a bit much, seeing as you don't have to meet any requirements to use it. Making it Bard only would at least require you to invest in UMD, like use of the scroll does.
Alternative Suggestion-
Reduce the charges to something more reasonable. Around 5-10.
Increase the price of the item, making it a bit harder to find.
Remove it entirely