1. Probably the easiest if you wanted to make them available for free, but charging would be more difficult. Perhaps you could set up some sort of e-commerce site - there are a number of open-source systems available - and bundle all the tracks in a single archive (such as a zip, perhaps with a password) and charge for that.
2. Probably not practical. You don't have any control over the pricing. Apple charges from US $.69 to $1.29, with no option for a content provider to set other prices. You probably wouldn't get a good response from people having to pay $.69 for a 4-second clip.
3. That's certainly possible. Whether that's your best option is difficult to say. You probably won't want to limit yourself to iOS devices only, so you'd have to create an app for each platform.
Another option would be to embed this all in an e-Book. If you have it all in InDesign, an Adobe PDF e-book might be the most practical since it could be cross-platform, either with the tracks embedded (if that's possible and practical) or containing links back to the clips on your web site, and then sell the ebook. If you don't mind targeting just iOS devices, you could instead use iBooks Author.
Hope this helps.