My media player Macintosh is nothing to write home about. It is an old (2006 model, I believe) 1.66 GHz Intel Core Duo with 1GB of RAM. It has an 80 GB hard drive, which I bought on sale from NewEgg after the old hard drive died in January. Any audio transmission solution needed to work within its limits, which I assure you are vast compared to the current generation mini, which is my primary work machine.
My home's 802.11g Wi-Fi sees low use during the day when my kids are at school. On an uncluttered system (i.e. the kids aren't all trying to watch YouTube videos at once) it's capable of some decent throughput but it's clearly not 802.11n. I record and convert all my video on a separate headless system in my office. My Wi-Fi connection needs to both feed my video to the player mini at the same time that it sends any audio via Skype.
Early investigation showed that I could read and play back video using QuickTime or iTunes while shipping out sound at the same time. EyeTV (my preferred video playback solution) plus Skype required too many system resources, producing video and audio hesitations during playback. Newer Macs with more RAM and better processors probably won't exhibit these limitations.
Setting up the audio feed
It took just a few steps to set up my audio feed. I needed to redirect my Mac's audio to Skype so I could listen to that audio from the iPhone or iPod touch. Here is a walkthrough of those steps for you. It shoudn't take you more than a few minutes to set up your own solution.
Download and Install SoundFlower. I started by downloading and installing a copy of Soundflower version 1.5.1. The dmg offered at the Google Code site includes an installer package that adds the Soundflower system extension. You'll need administrative privileges to install it.
Set Skype's Audio In. Once Soudnflower is installed, you need to set up your audio in two places. In Skype, open Audio settings and set your Audio input choice to Soundflower (2ch). Skype will now read its audio from whatever the Soundflower extension delivers.
Set Audio MIDI's Sound Output. Next, navigate to /Applications/Utilities and launch the Audio MIDI utility. Open the Audio Devices window. Choose Soundflower (2ch) and select Use this device for sound output from the action pop-up menu at the bottom of the pane. This choice sends normal system output that you'd normally hear through your speakers into Soundflower. A speaker icon appears to the right of Soundflower (2ch), indicating that it's your current choice.
If you're working on Leopard rather than Snow Leopard, use the Audio MIDI setup screen. Set the Default Output to Soundflower (2ch). It has the same effect of redirecting audio to Soundflower.
Create a second Skype account. You'll need two accounts for this Mac-to-iPhone audio solution. Skype accounts are free. Sign into your normal Skype account on your Macintosh.
Install Skype on the iPhone. If you have not already done so, purchase Skype (it is free, iTunes link) at App Store. Sync it to your iPhone, or as I did, your iPod touch. Sign into the second Skype account.
Call the iPhone from your Mac. Add your new account to Skype as a buddy and call your mobile device from your Mac. Accept the call on your iPhone or iPod. If you are using an iPod (and I mostly do, so to preserve the batteries on my iPhone for when I need to leave the house), you may be warned that there's no audio input on your end. Acknowledge the message and continue accepting the call.
Start playback. I've been using QuickTime X to play my video. Start playback and set it to full screen. (I hate that new always-on video controls overlay. Anyone know how to disable that display?) Front Row, iTunes, and QuickTime all seem to work well with this Skype-to-Skype playback solution on my older system.
Enjoy. Plug in your headphones, stick the iPhone in your pocket or into one of the cup holders on the treadmill, and watch as you exercise. I find it handy to keep a spare Apple remote around as well for starting and stopping the video. If you're using Front Row, the remote also lets you navigate through the menu system to pick and play your media.
My Skype-to-Skype solution is, admittedly, a little on the clunky side. At the same time, I've found the audio transmission quality to be excellent. If my call starts with any significant audio lag, I've found that I can just disconnect and try again until I have a connection with better response time.
This approach definitely does have a battery hit. Although you can run Skype with the iPhone locked, a one-hour run can go through a quarter to half of a battery charge. Again, this is a great reason to use a second unit if you have one on-hand, or to tether your iPhone to power while using the remote audio. Most treadmills have to plug in anyway. Adding a long tether to your iPhone and an A/C adapter can work if the tether doesn't trigger your iPhone to complain about unauthorized cables.
Remember to reset your Mac audio after your workout. You can leave the Skype setting be if you don't normally use Skype for conversations on the TV-based Mac. You'll want to switch the audio back to normal output in Audio MIDI until your next session.
Users on more modern Macintosh systems or on Windows may want to investigate Rogue Amoeba's $25 Airfoil system, which allows you to redirect audio from any application to AirPort Express and to iPhones and iPods running their (free) Airfoil Speakers Touch application (iTunes link). TUAW took a first look at Airfoil and Airfoil Speakers Touch back in April. You can download Airfoil for free and test it for up to 10 minutes at a time.