from bs4 import BeautifulSoup
html = '<meta property="og:audio" content="https://freesound.org/data/previews/611/611605_1648170-lq.mp3" />'
soup = BeautifulSoup(html,"lxml")
mp3_url = soup.find('meta',property='og:audio').get('content')
print(mp3_url)>>> https://freesound.org/data/previews/611/611605_1648170-lq.mp3