I have a digital piano and the sound is distorted when playing notes on the left hand (.i.e notes lower than the middle c)side of the keyboard note?

Sometimes the contacts under the keys get dirty and must be cleaned. To do this you have to disassemble the keyboard,
carefully remove the PCB under the whole keyboard, locate the not sounding notes, remove tre siliconic stripe with the contact and gently clean the inside black conductive rubber pills with a cottonfioc and alcool.
Also clean the contacts on PCB.

It's most likely to be dirty key contacts.
The contacts are conductive pads. They get "crud" on them over time, which then just needs to be cleaned in a certain manner.
Best to clean all the pads over the entire keyboard. It's not difficult for someone with experience, but it's not a job for a novice.
My guess is that the keys are toward the center of the keyboard.

That's very annoying! The keys press on to a contact strip running the length of the keyboard. Over time this wears out and notes can either stop being dynamic or stop playing altogether. You can buy the strip off ebay and have a go replacing it yourself - not too expensive. I paid someone 50pounds to supply and fit one to a 61-note keyboard.

It sounds like there may be an issue with the piano sounds on your Kurzweil sp4-7 keyboard. One possibility is that there may be a problem with the software or firmware of the keyboard, in which case you may need to update the software or contact the manufacturer for assistance. Another possibility is that there may be a problem with the internal hardware of the keyboard, in which case you may need to have it repaired by a professional technician. In either case.

Have a professional tune it for you. It may be that some strings have loosened/tightened.

This is a problem with one of the two contacts for that key. The contacts close at different times as the key is depressed and the time it takes between is measured as velocity which is used to control the loudness as in a regular piano. The contacts are conductive rubber pills. After a lot of dis-assembly you can clean them with 99% isoprphyl alcohol and Q Tips. Clean both the pills and the circuit traces on the board they are pressed against. Be very careful of the ribbon cables between the halves of the case.

I would check your fingering... You MAY be late in hitting a particular note and the using auto chord... this can cause the problem. Another thing is if notes are hit with different velocity that can affect the sound. Sometimes certain voices have VELOCITY switching which actually changes the voice depending on the velocity you hit a note.

All I can suggest is try a master reset. It could be a software bug or a dried out electrolytic capacitor in the power supply.

In playing fast you have to make sure to release the note fully and let the key come up. These keys have TWO contacts that are used in the scheme to measure note velocity. If BOTH contacts don't open, the note will NOT re-fire. The second symptom you have written would be a characteristic if one of the disconnect diodes has shorted. These diodes direct the strobe signals that scan the matrix of key contacts to avoid other octaves responding when multiple keys are depressed including those in other octaves. Servicing of the unit is required. It is unlikely you have the tools to DIY this problem. ONE THING to try is to tip the unit and gently tap it to see if some debris MIGHT have fallen in that may be shorting some of the matrix connections.