Could Adobe Air be the best solution for Mac and iPhone?
Well, first of all, Flash is not supported on the iPhone, so there is no Air platform for the iPhone (at least none of me know - correct me if I'm wrong).
Second, native Objective-C apps on the Mac will always outperform Air apps in how well they are tied to the OS. An example of this is TweetDeck. It's an awesome app, but it just doesn't feel like a Mac app (no meaningful menus, growl integration, etc.). Mac users are usually very specific about how their applications behave.
I would advise if you want to write Mac apps (and iPhone apps) to stick with Objective-C rather.
If your main concern is cross-platform (excluding the iPhone), then Air is an option worth considering.
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If you want to create an application exclusively for the Mac / iPhone platform, I would suggest using Objective-C instead. The strength of Adobe AIR is that it can be installed and used cross-platform with little hassle.
However, one of the downsides to this is that performance will never be as good as something directly related to the operating system.
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the problem with using middle frames is that you will never have the potential of the hardware, in other worlds you will not be able to use any feature that the Mac / Windows hardware can give you, you will always be tied to which frames you are given, and hope you can do whatever you need ...
specially in your music app.
regarding Adobe Air environment, it is recommended to use both Linux, Mac and Windows computers, but you cannot run it on iPhone because Adobe Flash is not supported ... Adobe is making a flash player for iPhone, but I believe it will support Air at least in the near future.
For iPhone, you can create a web app or SDK app, the latter, and if you use SDK 3.0, you will find many useful things to use, but you need to develop a complete app from sctrath.
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