No, 50W out is 4.17A which would be really pushing it for an EBAY buck converter made of pure 'Chinesium' + there is a high probability that at light loads either it or the laptop PSU will go into discontinuous mode and produce a lot of AF ripple, and at higher outputs, the PSU and the buck converter may 'beat' with a difference frequency in the AF range.
If you need 12V at over a fraction of an Amp, for audio of any better quality than a 'Tannoy' style PA system, don't start from a laptop power brick!