This only started happening recently, but it's a real pain. Sometimes my MBP just stops recognising any external speakers or headphones, nothing fixes it but putting the laptop to sleep and then waking it up again. My iPhone earbuds have also stopped being able to control the volume, not a big loss though because they are terrible. Is this a common problem? I noticed it only started appearing once Lion became available for sale... Maybe my MacBook is hinting at something seeing as I only have Snow Leopard.