The Senate Budget Committee yesterday opened hearings on how to balance the budget, just as the Senate as a whole was blowing a $366 million hole in its own spending restraint targets for 1981.

Chairman Edmund S. Muskie (D-Maine) noted, as he opened hearings, that the Senate had taken just a first step toward approving a 15 percent cost-of-living increase in education benefits for veterans beginning in September.

It was not by way of criticism but "in a spirit of cynicism," Muskie added, that he flet compelled to note that some of the sponsors of the spending restraint measures were suporting the veterans' aid increase. That increase, he said, would breach projected ceilings for veterans' benefits by $366 million in 1981 and $430 million in 1982.

Despite pleas for self-discipline from Muskie, the Senate Wednesday had voted 78 to 14 against his effort to kill by tabling the veterans' aid proposal sponsored by Sen. Alan Cranston (D-Calif). Yesterday it approved by voice vote Cranston's proposal, which Muskie called the kind of "heart-plucker" meaure that is usually responsible for spending excesses.

Those who had voted for the veterans' aid increase and were scheduled to testify before Muskie yesterday and today include Sens. Pete V. Domenici (R-N.M.). Dale Bumpers (D-Ark.), Gary Hart (D-Colo.), Birch Bayh (D-Ind.) and Charles H. Percy (R-Ill.).

Cranston contended that the 15 percent increase wouldn't even keep up with inflation, which has pushed living costs up 20 percent since benefits were last increased.

The Senate gave final approval to the veterans bill, 82 to 4, and sent it to conference with the House to resolve differences. The basic bill extends for three years the deadline for Vietnam veterans to take advantage of government aid for apprenticeship programs, on-the-job training and vocational education to train the hardcore unemployed for jobs.