Problems with loading software from tape (or virtual tape files) onto original Spectrum hardware are often caused by using incorrect tape leads and incorrectly identifying sockets. Different Spectrum models were supplied with different tape leads, all of which were designed to be used with mono cassette recorders.
Original tape leads
The ZX Spectrum 16K/48K and ZX Spectrum 128 both have a pair of 3.5mm mono sockets labelled EAR and MIC. These computers were supplied with a twin cable terminated at each end with a pair of 3.5mm mono jacks.
Note that the EAR socket is the input and connects to the earphone output of the cassette deck while MIC is the output and should be connected to the microphone input.
The ZX Spectrum +3 has a single 3.5mm stereo socket which is used for both tape input and output. The +3 was not supplied with a suitable cable but details are included in the manual. The tip of a 3.5mm stereo jack is connected to the MIC output, the ring connected to the EAR input (sleeve is ground common to both). The diagram in the manual shows a twin lead from the stereo jack to a pair of mono jacks for the earphone and microphone sockets of a mono cassette deck. The EAR input was disconnected on the +2A but can be restored easily by the addition of one capacitor on the motherboard.
The ZX Spectrum +2 and +2B have no provision for loading from an external tape deck so no leads were provided. The +2B can be modified to restore the same pinout (and hence tape lead) as the +3 but this requires cutting a trace on the motherboard to separate the tip and ring of the stereo socket.
Connecting to stereo equipment
Laptops, MP3 players, smartphones, etc. all have stereo outputs which shouldn't really have mono jacks plugged into them as this will short the right hand channel to ground. While this is unlikely to do permanent damage to your device it isn't ideal, however using the original tape leads in this way will often work OK.
Normal off-the-shelf stereo to stereo leads however are not suitable for any spectrum model.
On the 16K/48K and ZX Spectrum 128 the type of mono sockets used will only contact the tip and ring of the stereo jack plug, connecting the ring (right hand audio channel) to the Spectrum's ground and leaving the sleeve not grounded. The usual symptom of this is that the signal reaching the Spectrum is far too low to load.
Using such a cable with the +3 is even worse because while the right hand channel is connected to the EAR input, the left hand channel is connected to the Spectrum's output. Again this is unlikely to cause permanent damage, but connecting two audio outputs to each other is generally a bad idea!
For a 'perfect' tape lead to use with a device that has stereo input/outputs one should start with the relevant original tape lead pinout and replace the cassette ends with stereo jacks. Convention is to use the left channel for mono applications so the ground should be connected to sleeve and the signal to tip. If you want to route the MIC audio output to speakers (for listening to game audio etc) you may want to connect the MIC signal wire to both tip and ring so that the sound comes out both speakers.