You don't need expensive software. Here are the gold-standard tools for the job:
The Ultimate Guide to FLAC Gain Fix: How to Normalize Audio Without Losing Quality flac gain fix
Mp3tag can also manage ReplayGain information. You don't need expensive software
The most reliable method uses (FLAC utilities) to remove old tags and replaygain to recompute. flac gain fix
Go to > Volume and Compression > Loudness Normalization .
Once the scanner finishes analyzing the waveforms, a report window will appear showing the suggested volume adjustment (e.g., -3.12 dB or +1.50 dB). Click to save these adjustments directly into the FLAC metadata. Configuring Your Media Player