Slightly difficult to answer this one as there isn’t a single place the PCM data will be, or really an approved method of doing this. The RawData of a SoundWave should always be PCM data but it will only be loaded in editor builds generally and you’ll need to lock it before you can read from it. If you look at the Sound Visualisations plugin code you might see some examples similar to this.
If you are in a cooked game however, CompressedFormatData will be loaded instead. Then depending on whether the sound decompresses natively or at runtime, the PCM data may either be in the SoundWave’s RawPCMData pointer or decoded in chunks at regular intervals, which are part of a sound buffer that works differently on each platform/file type.