Encontrei uma resposta no site oficial que não requer instalação de software de terceiros. Você só precisa alterar as configurações de volume, como descrito:
There is a workaround described above that stands:
Set the volume on the Spotify player software to maximum. Then adjust the computer output volume back to a comfortable level. No more loud ads.
Problem lies in the fact that Spotify allows advertisers to adjust the player volume, against your will, when their ad is played, overriding the player volume setting you have set yourself. Naturally they set it to max - because they can!
When you set the volume of the player to max yourself, the ads cannot increase volume anymore higher than you now listen to your music at. Effectively you have created a ceiling to the audio volume.
This results music and ads be played at the uniform level. And you still can set the actual output volume to any level you like via the computer volume setting.
Tested. I can confirm this is an effective workaround, that Spotify cannot bypass without your consent.
