Firstly, as a learner of Japanese I have personally only heard them pronounced in the same way, and I have not been able to find any contrasting example on Forvo or Youtube (only a couple of searches).
(But I guess a possible explanation for any perceived deviations in pronunciation may come from the fact that the longer form of volleyball has another accentuation pattern, which perhaps may influence some speakers to pronounce the short form of volleyball (slightly) different than ballet.)
All resources I have checked also give the pronunciations as the same. I have taken the liberty to convert the various accent visualisations to the format of Japanese.SE, see below.
Kenkyusha’s New Japanese—English Dictionary (4th ed., 1974):
バレー{HLL} ballet, volleyball
バレーボール{LHHHLL} volleyball
English Wiktionary (as of 2017-02-02):
バレー{HLL} volleyball, ballet
バレエ{HLL} ballet
バレーボール{LHHHLL} volleyball
OJAD (as of 2017-02-02):
バレエ{HLL}
バレーボール{LHHHLL}