Basically, то что предложил
SoreMix:
@client.event # https://discordpy.readthedocs.io/en/stable/api.html#discord.Client.event
async def on_voice_state_update(member, before, after): # https://discordpy.readthedocs.io/en/stable/api.html#discord.on_voice_state_update
if not before.channel and after.channel: # https://discordpy.readthedocs.io/en/stable/api.html#discord.VoiceState
await member.add_roles(member.guild.get_role(748879395706999708), reason="Voice role")
# https://discordpy.readthedocs.io/en/stable/api.html#discord.Member.add_roles
# https://discordpy.readthedocs.io/en/stable/api.html#discord.Guild.get_role