I don’t know about speed tool…but I never use mp3’s. I get clicks and pops. I always convert mp3’s to AIF or Wave at 48k. my timeline is set to 48k so I never have to render audio.
is your sequence set to “render in 10 bit high precision YUV”.? you might try setting it to “render in 8 bit YUV”. also i would render in ProRes…not HQ.
i tried the same thing without problems. selected both tracks/modify/uncheck stereo pair. i adjusted left channel to -inf. and the other channel did not follow..it stayed at 0.