Sorry to be replying so late, but it appears that the query of this thread was more or less left hanging.
Bielarus’ does indeed have a chant tradition. It is similar to that of Ukraine and Carpathian Rus’, but with the inevitable local distinctive characteristics. The Suprasl’ Irmologion, written ca. 1600, is one of handful of irmologia dating from the last years of the sixteenth and first years of the seventeenth century that are the oldest sources of Orthodox liturgical singing in staff notation--the square-headed Kievan notation, of course--and a very important historical source for Orthodox church singing.
Why, then, is this tradition so little known? Prof. Jasynovs’kyj's superb catalogue of manuscript Irmologia lists some 1100 manuscripts, but the Bielarusian mss among them are only 75 in number, a startlingly low figure.
The reason for this, I fear, is discreditable to Orthodoxy. As many members of this forum will recall, the Poles took control of Bielarus’ (from the Lithuanians) in the sixteenth century, just at the time when they were abandoning the policy of religious tolerance that had previously characterized the Polish-Lithuanian state. In the seventeenth and eighteenth century, the Unia enjoyed considerable success in Bielarus’. The partitions of Poland in the late eighteenth century brought Bielarus’ under Russian rule, but many of the Uniates were unwilling to return to Orthodoxy. After the Polish Rising of 1830-31 (in which Lithuanians and Bielarusians participated along with Poles), St Petersburg was determined to wipe out this Unia, and there was widespread resistance from the Bielarusians. The result was harsh repression: several villages were the scenes of massacres, a large number of priests were sent to Siberia, and so on. One feature of this repression is particularly relevant here: Bishop Siemashko, who presided over the (largely involuntary) return of Bielarus’ to Orthodoxy, ordered the church books found in Uniate churches burnt.
These books differed in a number of ways from the Russian norm, not because of any latinization or Uniate influence but because they preserved old local Orthodox tradition that often preserved either old Orthodox traditions lost in Russia or local Orthodox usages differing from those of historical Muscovy. Among the books destroyed in the massive vandalism were, of course, the chant books. And as in Ukraine, once the Bielarusians fell under the Holy Synod of the Russian Church, they were dragooned into conformity with Russian usage.
This is the major reason why the old Bielarusian tradition of Orthodox church singing is largely lost today.