I've never heard of anyone getting Linux running under hard real time constraints. You can get it pretty good (excellent for real time audio, for example), but can never be 100% sure you're going to meet the deadline. The people I've talked to who tried said by the time you strip out enough of the kernel to approach hard real time, you've lost enough of the advantage of using Linux that you may as well switch to an RTOS.
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Real time audio only if you use a large enough buffer and a hardware component to clock the data out.
It's all about the latency guarantees and that means that you're going to have to inspect each and every path through the code for length. With the complexity of the linux kernel that's a pretty tough job and I suspect that anything lower than a few hundred milliseconds (guaranteed!) is out of the question.
I built a little controller using linux that required hard real time during a long (multiple minutes) of operation and the way I hacked it was to simply disable all interrupts and recover the various drivers as good as possible once that phase was over. It worked well but was mostly deaf to input during that time except for polling one 'stop' switch which would cause the machinery to coast down to a halt after which interrupts would be enabled.
Good enough for tinkering but I certainly would not bet anything in production on that strategy.
Real time is hard, soft real time is hard enough (without guarantees but with a best effort and a very large fraction of the deadlines satisfied), hard real time (no misses at all, guaranteed) is hard for a kernel of any complexity.