If teaching is your back-up option as I think you've indicated, I would not worry too much - better to focus on what your first choice is first! You don't want to go with your second choice degree just because it fits with your back up.
In terms of PGCEs, you'd have no problem being qualified for a Primary PGCE and most likely not for a Post-Secondary PGCE.
However, what colleges will allow you to teach will vary. Some might insist that your degree is in the subject so this could depend how much Biology you do as part of your degree.
Others will be more lenient - my A Level Italian teacher was a French grad who did a year of Italian and a year in Italy with regular trips back there to brush up.
If you're looking at secondary teaching and you're not heart set on an Audiology career and think teaching is likely.... then I'd suggest emailing a couple of universities to find out how they'd view it. It can't hurt to ask.
In final year of my degree (French, German and Japanese) I started looking at PGCEs and was pretty shocked to find out I wasn't eligible to apply at any of my 5 choices! I'd just assumed I'd be okay because I was on track for a first, could offer 3 languages at degree level, plus Italian at A Level and you only need an A Level to teach KS3 langs, and I had good work experience. But the 5 unis I emailed to ask about admissions said they wouldn't even look at my application because either I didn't have 50% of my degree in my main lang... or I hadn't lived in the country of my main lang (I spent my year abroad in Japan and Japanese PGCE's are pretty non-existent). Luckily this was just something I was considering doing, so I got over it
But if I'd have known since prior to uni that teaching might be my goal I'd have been pretty devastated to find out my first class degree wasn't going to get me there just because of the combination I'd chosen!