There's only one album where XZ beats any codec, and that's on mono material with lots of silence.
LZ77 does very well when you have repeating sequences. Silence falls into that category. Although xz does use a "delta" filter when applied to audio data. It probably allows the LZ77 model to find the patterns.
XZ can do particularly well if you use a custom filter chain. I can get pretty good ratios (though still not as good as flac -8) when using XZ's ability to use a custom filter chain. Specifically, I use
xz -vvk --delta=dist=4 --delta=dist=4 --lzma2=dict=128MiB,lc=0,lp=2,pb=2,mode=normal,nice=273,mf=bt4,depth=1024 Audio_file.wav
but I change the dictionary size to be the smallest value that is either of the form 2^n or 2^n + 2^(n-1) that's larger than the file I'm compressing, because anything larger is unnecessary and those are the values that XZ Supports.
Note that with delta, you can specify the distance, which is extremely useful because each sample is 4 bytes long (16-bit stereo, adjust for other formats) so the corresponding byte would be 4 bytes away. Using delta twice improves the ratio further in every audio file I've tried it on, but I don't entirely know why. For some reason, three delta filters consistently performs worse than two, even though two consistently performs better than one. Someone else will have to explain this one to me.
Also note the values I'm using for lc, lp, and pb. (The other non-dict values are just max settings.) It's easier to explain if I quote the XZ manpages:
By using lp=2 and pb=2, I set LZMA2 to assume 4-byte alignment for everything, which is exactly how I want it for 16-bit stereo samples. I'd change these to 1 for 16-bit mono, 24-bit stereo, and 8-bit stereo. (24-bit stereo only contains one factor of two, but 16-bit stereo contains two; that's why it's lower for this one). The choice of lc=0, lc=1, or lc=2 is not critical: I tried all three on several samples and got nearly identical results (as in within .001 ratio), better or worse depending on the samples.
So there you have it, your guide on how to better compress your wav files with XZ. Final answer: Use FLAC or any other audio-oriented program. FLAC compresses better and also has much faster compression and decompression; FLAC decompressed around 6x faster than XZ and compressed around 20x faster.