when pronouncing this should the 'l' be sounded or not. I have heard it both ways on forvo, so what is recommended way
Both the 'L' in 'mladá' and the 'L' in 'holka' should be pronounced.
Holka is a slang word, no? Slečna or dífka would be more grammatically correct.
"holka" is not really a slang word, just a little less formal. It's still correct and acceptable in most situations. Also it's "dívka", not "dífka". "slečna" isn't an exact synonym for dívka/holka, it's closer to the English word "miss".
"Mladý" is for the masculine gram. gender: "mladý muž" (young man). "Mladá" is used for the feminine gram. gender: "mladá dívka" (young girl). Remember that while the gram. gender usually coincides with the real gender when talking about people, it's not always: "děvče" has the neuter gram. gender, and the result is "mladé děvče".