If there's a difference in sound between playback and rendering, it could be worth playing with the quality settings for both playback and rendering.

Playback settings are under Options > Preferences > RealTracks:

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And render settings are in the export window under Audio > Export Song As Audio File:

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One thing to note is that low and high quality are somewhat subjective, and that some people prefer the sound of the low or medium setting over high. However, the more important thing in this case is consistency between the playback and export sounds, so I would recommend the High setting for both playback and export.

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