wrap the matrix with the the Flatten function
M = {{{1}, {2}, {3}, {4}}, {{5}, {6}, {7}, {8}}} 
Map[Flatten,{{{1}, {2}, {3}, {4}}, {{5}, {6}, {7}, {8}}}] {{1, 2, 3, 4}, {5, 6, 7, 8}}
In Mathematica this is known as "nested" Lists. Flatten removes all wrappings of the List function until only one remains