;) I agree that it sounds like cosca. But it also sounds exactly like costa should sound. As for not pronouncing things well the key thing to remember is ... Spanish is NOT English. Except in some rare cases where it looks like they gave the machine the wrong text to pronounce, it is perfect! Just like Spanish. Or so my Spanish speaking friends tell me.
In real life, speakers use the additional context of a conversation to resolve ambiguity. Our decontextualized snippets are difficult to understand. I wish you success in your studies.