simple translation

Here's what I make of it:

Nos tamen has voces sedulo distinguimus,
We nevertheless carefully distinguish these sounds

ne imposterum (quod vulgo fieri solet)
lest hereafter (which commonly happens)

pro iisdem habeamus instrumenta prorsus diversa,
for the same [sounds] we have altogether different instruments

nec porro observationes confundamus,
nor, furthermore, do we confuse observations,

factaque hac confusione conclusiones erroneas ex iisdem deducamus.
and because of this confusion draw erroneous conclusions from these same [observations].