Maps a few of the Fn keys on the Microsoft Sculpt Ergonomic Desktop to perform their media key equivalents. Install KeyRemap4MacBook, set your keyboard to the Fn mode and edit the "private.xml" as explained in https://pqrs.org/macosx/keyremap4macbook/xml.html.en

private.xml

<?xml version="1.0"?>
<root>
	<devicevendordef>
		<vendorname>MICROSOFT</vendorname>
		<vendorid>0x045e</vendorid>
	</devicevendordef>

	<deviceproductdef>
		<productname>MY_MICROSOFT_KEYBOARD</productname>
		<productid>0x07a5</productid>
	</deviceproductdef>

	<item>
		<name>F1 to Play/Pause</name>
		<identifier>private.f1</identifier>
		<device_only>DeviceVendor::MICROSOFT, DeviceProduct::MY_MICROSOFT_KEYBOARD</device_only>
		<autogen>__KeyToKey__ KeyCode::F1, KeyCode::VK_CONSUMERKEY_MUSIC_PLAY</autogen>
	</item>
	<item>
		<name>F2 to Mute</name>
		<identifier>private.f2</identifier>
		<device_only>DeviceVendor::MICROSOFT, DeviceProduct::MY_MICROSOFT_KEYBOARD</device_only>
		<autogen>__KeyToKey__ KeyCode::F2, KeyCode::VK_CONSUMERKEY_VOLUME_MUTE</autogen>
	</item>
	<item>
		<name>F3 to Volume Down</name>
		<identifier>private.f3</identifier>
		<device_only>DeviceVendor::MICROSOFT, DeviceProduct::MY_MICROSOFT_KEYBOARD</device_only>
		<autogen>__KeyToKey__ KeyCode::F3, KeyCode::VK_CONSUMERKEY_VOLUME_DOWN</autogen>
	</item>
	<item>
		<name>F4 to Volume Up</name>
		<identifier>private.f4</identifier>
		<device_only>DeviceVendor::MICROSOFT, DeviceProduct::MY_MICROSOFT_KEYBOARD</device_only>
		<autogen>__KeyToKey__ KeyCode::F4, KeyCode::VK_CONSUMERKEY_VOLUME_UP</autogen>
	</item>
</root>