When you hear a soft hiss / buzz / whine where DMR received audio should be heard, that indicates the DMR DSP has 'hung'. Sometimes changing channel and coming back to the original channel fixes this and DMR audio is heard again. It seems to be a problem only observed on hotspots or MMDVM based repeaters, never on 'real' repeaters with a reliable wide area connection.
There are a few other things you can do which may also help:
1) Confirm your SCT3258 DSP code is the latest: V2.01.05NJ. Check this in the radio menu using Menu > Settings > Device Info > scroll all the way to the bottom to see Sct 3258 Ver.
If you need to update your version, or feel you might benefit from reloading the DSP firmware, download the DSP update pack and instructions from:
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2) Update your 868 or 878 firmware to the latest version. Presently, the most current versions (early Aug 2019) are 868: 2.35 and 878: 1.13
3) Play around with the DMR channel type in the radio menu. Menu > Settings > Chan Set > DMR Mode. Choices are Simplex, Repeater & Double Slot. Simplex is normally used where TX & RX frequencies are the same, but counter-intuitively may also be needed for duplex hotspots. Repeater is used for, uh, repeaters. Double Slot isn't typically used.