From: Gale Andrews <gale@audacityteam.org>

Libsamplerate has five choices in 
Audacity:

ZOH Interpolator
Linear Interpolator
Fastest Sinc Interpolator
Medium Sinc Interpolator
Best Sinc Interpolator

Do we know which of those is used for Time Track rendering and which
for Time Track playback?

Also perhaps the Manual needs to mention that to be sure which
libsamplerate quality is used for Time Tracks, initialise Prefs. 

Copy of  some info I just sent to Gale, off-list:
 
Here's the situation for var-rate @ r12165:

Render Play
libsoxr Var-rate-q* Var-rate-q*
libresample High-q sinc Fast sinc
libsamplerate Best sinc Fastest sinc

* libsoxr currently has a single, dedicated quality level for var-rate resampling.

Cheers,
Rob