iMac, Volume

On my 27" iMac: I know that total volume out when using iTunes depends on the iTunes volume setting and the system volume setting. But with you tube, Prime Movies, Netflix, etc there is no volume adjustment and the system volume is simply too low. Other than external speakers how can I increase the volume. The iMac is capable because some streaming sources are very loud and I must turn the volume down on the iMac. Like intro's to a movie, yet then the dialog is so low as to be unintelligible at max volume. Are the newer iMacs any better? Mine is a late 2009.