Parallel Processing in C# to SQLite

We have a working C# parallel processing project. But it has problems. It takes a longish (1 hour or so) WAV files and processes them in parallel. The trouble is, some work was done recently on the parallel processing and that has slowed it down so that it is now slower than a single thread even with 8 threads. It runs at about the same speed regardless of the number of threads. It was originally at best 2 x faster than one thread with any number of threads available.

The processing is done in C# and it writes to the SQLite when it is finished. It has a timer as part of the software.