The Sale of Goods Act is quite clear on retailer responsibility. Unfortunately, almost all retailers will refer you to the manufacturer, many of whom have a swap out warranty policy. Most consumers opt for this as it's more convenient than having the retailer organise the repair, leaving us with no monitor.
Nonetheless, the liability is still on the retailer, and as this is the second one to fail, you might want to tackle them again. When I has a similar problem with a Viewsonic monitor, Scan agreed to take it back, give me a refund and sell me a new monitor (Eizo). They arranged for the new one to arrive the day before they collected the Viewsonic, to minimise inconvenience.
Don't have any knowledge of the sound card, sorry.