这是获取此HTML字符串中唯一的MP3 href的正确xpath查询语句吗？

I am having some trouble with an xpath query I am using in my code and thought I would throw this out there and ask the community if this query is in fact correctly formulated for its intended purpose. As shown in the code below, I am looking to grab the only MP3 URL in the a tag's href attribute. This entire bit of HTML is fed into xpath as a string.I am having some trouble with an xpath query I