What you really need is a 'virtual cart machine' programme, such as we use on the radio.

There are some free ones around, and some are better than others. The one that would probably do best for what you need is 'Jazler Show'... it doesn't assign key presses, you have to point with the mouse, but it can store an almost unlimited number of 'carts' for you to choose from. Make sure you pick Jazler Show, which is the free one!

If you don't like that one, do a search and find one you do like.