http://apple.stackexchange.com/questions/6173/can-i-play-audio-from-my-iphone-on-my-mac

Simple, and simply surprising:

1. Plug in your iPhone to your Mac using your Lightning Cable (doesn‘t work with 30 pin iPhones, sorry. [Yes I know you said 3GS])
2. Launch QuickTime Player
3. Choose File> New Audio Recording
4. Click the hard-to-see down arrow next to the Record ‘button‘, choose iPhone under ‘Microphone‘
5. Play your music, game whatever on your iPhone, sound will come thru your Mac.

No need to hit record, the sound will play even if you don‘t record.

Mac OS 电脑播放 iPhone音乐