Proposal to amend GRBP/2026/4. Amendments to UN Regulation No. 51 include updates to the Table of Symbols in paragraph 2.24; new definitions for Active Sound Source, Enhanced Exterior Sound (EES), and Vehicle Master Control Switch; specifications for EES operation requiring driver intentional action and default deactivation; sound limits complying with UN-Regulation No. 138 below 20 km/h and this Regulation from 20 to 80 km/h; transitional provisions allowing extension of existing approvals for 24 months; modifications to Annex 7 control ranges, anchor points, and slope calculations for electrically propelled vehicles; and updates to Annex 1 and Annex 3 regarding EES systems. GRB-68-03 commitment to add EES guidance document is confirmed.