Why it is called as Sounding ?
Here the meaning of sounding is the action of measuring the quality of the channel. Sounding reference signals (SRS) are transmitted on the uplink and allow the network to estimate the quality of the channel at different frequencies. It's the reference signal for eNodeB which is send by the UE to inform about the channel quality.
Demodulation reference signal (DMRS) in uplink transmission is used for channel estimation and for coherent demodulation which comes along with PUSCH and PUCCH. If DMRS is bad or by some reason not decoded properly by base station , PUSCH or PUCCH will be not decoded as well.
SRS is an configurable parameter so yes it is an optional but DMRS is not optional like SRS.
Difference between SRS and DMRS.
I hope I answered your all queries.