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You can add reverb for free using online tools or software that come with built-in reverberation effects. Some options include:

1. Audacity: A free, open-source audio editing software that includes various effect plugins, one of them being reverb.

2. Ozone Immersive EQ: iZotope offers a limited but functional version of their Ozone Immersive EQ tool for free. It provides a easy way to apply reverb to your audio.

3. GarageBand: If you’re an Apple user, GarageBand already comes installed on your device and has various built-in effects, including reverb.

4. Cockos Reaper: It’s a digital audio workstation that has a fully functional trial period, and it also includes reverb effects.

5. Casio Cakewalk by BandLab: A DAW that is free and has a reverb