To continue from Nicholaz's answer this reveals some wake events in the event log that I missed out, since they were not errors:
To find out what caused your computer to wake from sleep, do the following:
Open Event Viewer by clicking the Start button , clicking Control Panel, clicking System and Maintenance, clicking Administrative Tools, and then double-clicking Event Viewer. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
Expand Windows Logs, and then click System.
In the Actions pane, click Filter Current Log.
In the Filter Current Log dialog box, in the Logged list, select the time range.
In the Event sources list, select Power-Troubleshooter, and then click OK.
In Event Viewer, in the System pane, select the date and time for the event that you want to view.
On the General tab, view the Wake Source for the event.
After inspecting the filtered results, I see plenty of messages like so:
The system has resumed from sleep.
Sleep Time: 2010-02-24T23:42:44.283678200Z
Wake Time: 2010-02-24T23:43:10.233041900Z
Wake Source: Device -USB Root Hub
However, I've also spotted this rather annoying "unknown source" variant:
The system has resumed from sleep.
Sleep Time: 2010-02-20T22:41:30.687040200Z
Wake Time: 2010-02-21T10:07:54.488171000Z
Wake Source: Unknown
I will update my answer if the solution is disallowing this device to wake the computer.
Update:
I have resolved my issue!
Turns out that the "USB Root Hub" source is a little misleading; it was actually both my keyboard and mouse that were waking up my computer (without being touched/pressed). The solution was to uncheck "Allow this device to wake up the computer" on both the mouse and keyboard devices (selecting them independently does not resolve the issue).
Perhaps updating my mouse/keyboard drivers will fully resolve the issue.
Yes, you can have the Thinkpad go into hibernation from the sleep state. If the machine in connected to power, you can make it work by setting for sleep in 20 and hibernate in 25. This causes hibernation to kick in 25 minutes after sleep has started.
However, if you are running on batteries (as most people are who wants a sleeping machine to hibernate), it does require a BIOS configuration change. In the BIOS choose Config, Power and enable "Timer Wake with Battery Operation".
Warning: Lenovo do not recommend you do that as it may damage your hard disk if you are moving the machine around when it decides it is time to go from sleep to hibernate. Also you have to be sure about other scheduled tasks so they don't risk waking the machine and draining your battery.
Best Answer
You should make sure you've got the latest drivers for your sound card and system board as well as the latest Windows patches.
Power states and drivers enjoy a tenuous relationship at best, and the first step of troubleshooting any issues that appear related to changing power states should always involve making sure the drivers of any and all affected systems are up to date.
Next, check to see if the Windows Audio service is running after after a power state cycle. Start -> Run -> "services.msc" Enter. Check that Windows Audio service status is "Started".
Also, when you're playing audio and there's no audio output, double-click on the volume control in your taskbar and see if the audio level indicator is moving indicating the system "sees" audio passing through the volume control.