The Open Goldberg Variations project produces audio files in various formats, bit depths and sample rates including WAV and FLAC at 24bit.
While not free of license, Carmen Gomes - A Fool For You - Compare Formats is a free of cost audio track available in multiple formats (DXD, PCM, DSD and FLAC) at various bit depths and and sample rates, choose “ALL TRACKS” to download the entire/complete bundle. The “Compare Formats” bundle is referenced by many as their source used when comparing formats and testing DACs, although, I’m of the opinion that the quality of the master is a little questionable.
There is plenty of license and royalty free hi-res music available, a simple web search should turn up some results.
Assuming that the WAV and FLAC files have bee derived from the same source and are straight conversions with no additional processing, if you can hear a difference then I would suggest there is something wrong within your audio chain.